GSHPs get their energy from either horizontal "slinky" coils or very expensive deep vertical bore holes.
Another option is straight horizontal pipes, which is what we have. Slinky pipes are a good solution if you have a limited area in which to place the pipes. We did not have that limitation so straight pipes were a better bet. Yes, it needed a longer trench (we have 800m of pipe) which can add to the cost of digging, but you don't need such a wide trench which offsets that a bit.
Standard emitters, i.e. radiators, need to be oversized wrt. those used for gas CH as the temperature is significantly lower.
We were lucky in that our existing (oil-fired) central heating already ran at a lower temperature - maximum around 55C - so we did not need to change any radiators.
Heat pumps also produce slow, gentle, continuous heat rather than the full-on anger of a gas CH system, thus they perform far far better in well-insulated and non-draughty buildings. They tend to run continuously and do their own thing - not something that you have on for an hour in the morning and an hour or so at night.
This can be a tricky one to get your head around if you are used to the approach typical in the UK of running the heating for only part of the day. That can actually cost more with a heat pump because when it comes on after being off for a few hours the back-up resistive heating (basically a big immersion heater) can kick in and that is more expensive to run. Add in low-rate night time electricity into the mix and it is not always obvious how best to set the controls.
I’ve done lots of research but there are no tried and tested solutions except to rip out all the internal lath and plaster and rebuild with insulated walls.
Our house is a totally different construction. Originally just solid log walls it later had wooden ship-lap style outer covering and internal batten and fibreboard insulation added. I have had the same problem though: I cannot find reliable & understandable information about how to improve the wall insulation. There are lots of horror stories about old log buildings being ruined by modern insulation causing problems with damp & mould
GSHP solution seems the most reliable, the most costly, and most complex to install.
Generally reliable yes, although ours has not been entirely trouble-free. Costly to install (but not to run). Complex to install? Not complex ecactly - there is nothing complex about digging a trench and putting a pipe in it. Maybe involved would be a better word.
ASHP is cheaper and simpler to install, but less efficient. For a given amount of heat it requires more external power. It is also somwhat sensitive to ambient temperatures.
I do wonder if this might vary according to the time of year. In winter, when ground temperature is significantly higher than ambient I expect GSHP is far better, but what about in spring and autumn when ambient temperature is above ground temperature? Is the higher-temperature heat source enough to offset any lower efficiency of ASHP?
Solar water heating - fairly simple, easy to retrofit.
I fitted evacuated tubes and they work well. Most of the time we put their output into the heating system though, not to hot water. That is a better use for us. Also we have a problem in that the only real location suitable for the tubes is a long way from the hot water tanks so there is heat loss along the way.
It is not a simplistic - "fit a heat pump".
Effectively that is what we did. We fitted a heat pump and connected it into the existing heating system. We didn't change anything else; we didn't even remove the old oil burner which still exists "as a backup".
Ok all this is ok if (1) you are still quite young and can afford to pay off instalation over many years
That is a very good point. Older people and/or the less well off can really be disadvantaged. If heat pumps were the best solution for all then everyone would fit them anyway and no mandate would be needed.