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That [before you edited it] all sounds reasonable to me.

I suspect you'll need two completely different sets of rules depending on where your energy is coming from, so do a search through here for stratification. You might find that you want to add a pump to circulate the hot water within the tank when the energy is from PV, and/or you might want to add temperature sensors at selected levels down the tank so that you know how much hot water there is in the tank when you're paying for the energy.

Our normal assumption is that the information flow is from the Arduino to the Base to the Web, yours is exactly the opposite but as the RFM69 radio modules are both transmitter and receiver (though half-duplex working only), that should be OK.

[Note: to avoid confusion, please don't refer to the Shield as an "emonTx" - that's a different beast that has it's own processor, which the EmonTx Shield doesn't have.]

You also might want to (and reading between the lines you have already) read up about the PV diverter. Given some local generation, that will mop up surplus power in a controlled fashion. How effective that is depends significantly on the properties of your supplier's energy meter and what your import and export tariffs are.

"You might find that you want to add a pump to circulate the hot water within the tank when the energy is from PV"

I am not sure why a pump would be required. The heating element is at the bottom of the tank. Stratification is actually a good thing because the hot water rises to the top of the tank to be used which means the whole tank does not have to be heated in order to have a comfortable shower. However, if I do have access to a good price, or the sun is shining and I have lots of Excess PV generation then the whole tank can easily be heated to full temperature because the heating element is at the bottom. 

"Our normal assumption is that the information flow is from the Arduino to the Base to the Web, yours is exactly the opposite but as the RFM69 radio modules are both transmitter and receiver (though half-duplex working only), that should be OK."

I don't understand the comment about "half-duplexing", but I am sure that it will become clear to me once I have the hardware and I can start experimenting. 

"You also might want to (and reading between the lines you have already) read up about the PV diverter. Given some local generation, that will mop up surplus power in a controlled fashion. How effective that is depends significantly on the properties of your supplier's energy meter and what your import and export tariffs are."

Yes, PV diversion was my first goal but then this company started up which is going to be selling power at spot prices. This potentially means that solar won't be economically viable for a few more years because a micro-controller can be used to demand power at prices that are cheaper than solar panels are able to provide. I will eventually get solar panels even if the economics does not make complete sense because it is the right thing to do. 

I recently got a quote for a 3kw solar array which would provide power at 18c per kWh for the next 15 years. 

This electricity companies prices at night can be as low as 13c kWh. Based on their information, the price is less than 18c/kWh about 37 percent of the time or 8 hours per day. To raise my HWC from 20deg to 60 deg would require 10.2kWh of electricity plus 2kWh for standing losses. So I would need 4 hours of electricity per day below 18c kWh to be cheaper than solar panels. 

I don't understand the comment about "half-duplexing", but I am sure that it will become clear to me once I have the hardware and I can start experimenting. 

A familiar example of half-duplex operation is the two-way voice radio. i.e. user A must stop transmitting and give user B the go-ahead to transmit before user B can transmit and be heard by user A. (and vice-versa) In short, only one user can transmit at a time.

The ordinary telephone is a common example of full-duplex operation. Both parties can transmit and receive at the same time.

"A familiar example of half-duplex operation is the two-way voice radio. i.e. user A must stop transmitting and give user B the go-ahead to transmit before user B can transmit and be heard by user A. (and vice-versa) In short, only one user can transmit at a time.

The ordinary telephone is a common example of full-duplex operation. Both parties can transmit and receive at the same time."

Ah ok, that's simple enough. What implications does this have for sending data from the Base Station to the EmonTx Arduino Shield? How do they manage their two way communication?

"How do they manage their two way communication?"
Simply, you can't have both ends trying to transmit at the same time. If you know the shield transmits every 20 s (say), a good way would be to wait for that, then transmit in the opposite direction. It's up to you to define a workable protocol.

"Stratification is actually a good thing"
That's been questioned here in the past. Hence "Might...want to...". You can search and read the arguments if you wish.

I would put as much intelligence in the Pi as possible, and leave the Arduino to concentrate on switching, so every 2 mins (say) you update the Arduino with a "heat/don't heat the water" command, and leave it to get on with it. It means you might pay or lose out on 119.98 seconds' worth, though I wouldn't want to issue the command on every mains cycle or even every second. It depends of course on how often and when the prices are published.
(If you're going to transmit a lot and often, you need to check the local regulations for that RF band. Here, I think you're allowed to transmit for 1% of the time.)

Q2 & 3.
You need to study the sketch for that. An example of a sketch that both receives and transmits is one of the GLCD ones.

Q5.
Basically, as long as it's built as a PV Router, you just turn the output hard on or hard off until such time as you go proportional. It's pretty much like the thread: "switching fixed load on when surplus pv passes programmed amount." was asking.

Q6.
Maybe not. When you buy the "voltage output" sort, they have an internal burden resistor instead of zener diodes for protection. As I understand it, that's the only difference. So if you envisage a need for flexibility in the future, I'd get 3 proper current transformers and change the burden resistor on the Shield - that way you can scale it for whatever current you want (within the CT's limits of accuracy of course). Yes, the burden is the big SMT one behind the connector (and do make sure you assemble the connectors on the 'wrong' side as per the photos) and yes, you unsolder and remove the SMT one and there are holes for a wire-ended component instead (not as well as).