Hiya,

What is the heating requirement of the home ?

gg

The perceived wisdom is to separate CH from DHW as the demands of each are different temps. and at different times. The 300lts is not much to supply both CH and DHW. ASHP loose efficiency at DHW temps. so perhaps a separate DHW tank on E7 might make sense. If FIT requires DHW from a HP then get a tank with a coil and don't / minimise usage.

Gledhill are in the business of selling their products, this is not to say that their product is bad, just that it may not be the best for your situation. (This applies to any TS in the 300lt size running CH and DHW)
The bottom 1/3 of the tank being the buffer for the CH - 100lts of HP heated water goes nowhere when considering CH. IMO at best it could act as a mixer for multiple heat inputs. When the CH comes on the 100lts would probably get the pipes warm and not much else.

Do you plan to run the ASHP with solar PV or E7?

You say there is no space for a propane tank. Have you considered a couple of 47kg propane bottles? 47kg propane is about 650kWh which at your projected heat demand of 1.4kWh at delta 20deg. gives about 19 days continuous use. (please check my figures) if you have a wood stove as well then this might be an option. The advantage would be a main stream gas boiler which would probably be a much cheaper installation cost than a (sophisticated) TS and an ASHP. (how noisy are modern ASHPs?) E7 is off peak - yes? heating won't be needed off peak so the CH will be at full rate. E7 would be fine for a standard DHW immersion tank.

The 47kg propane cylinders can be stored against a wall with 1m clearance to an opening window (plus a couple of other requirements) so not that difficult to place.

I can't understand the paranoia about unvented cylinders in the UK. Over here there are everywhere, (no unvented cylinders), you go to the shop, buy one of the size to suit up to 200lts, take it home, hang it on the wall and enjoy. (200lts costs about 300GBP) and you don't hear of failures. The hard water means that every 5 years or so you need to take out the heating element on its base plate and replace it, a DIY job.

Can you run the CH on the ASHP without the TS so just use the vented TS for DHW? Then have a simple vented DHW cylinder with a PHX. That should be (much) cheaper than a sophisticated TS

https://m.youtube.com/watch?v=jbreKn4PoAc

Edit: this is the discovery channel episode where they heat an unvented cylinder until it explodes. The shell acts like a rocket and blows through the roof and high in the air.
They don't explain the physics - the pressure holds the water in a superheated state until the cylinder cracks and releases pressure, then the water transforms violently into steam in a BLEVE explosion. A vented cylinder cannot do this as the water cannot get superheated.

Posted By: djh
The only complexity in a thermal store is the control unit and pump needed to feed the PHX with hot water at the correct rate to ensure a stable output temperature, which would be needed in any case.

This I tried to address by including a Danfoss RAVK valve at the primary side of the PHX right at the pump output. The temperature sensing blob sits at the secondary side, so the thermal store pump gets pinched off harder the hotter the hot water becomes. It works fairly well to even out the heat draw off, although hot water temperature still gets quite high at low flow conditions, as the RAVK is designed never to close fully.
Alternatives would be to include a mixing valve, or a solution with lots of electronics doing the same thing as the RAVK, but better (and less reliable).