@dr_dongle If you wish to consider underfloor insulation, Q-Bot can use a ‘robot’ to spray under the floor with just 200 mm of clearance https://q-bot.co  

Not cheap but we had ours done about 3 years back and have found it beneficial. They can gain access through a small hatch or create one for you if required and they are very tidy workers. They created an access trap in our understairs cupboard and left everything very tidy afterwards. Regards, Toodles.

@Toodles That's very interesting to hear, thanks.  I had seen their adverts but experience of underfloor voids with their usual quota of holes, bumps and builders' detritus made me sceptical. There is also the issue of sleeper walls and external walls that are now internal. (this house was built in stages and totally remodelled at least once). That's not a reason not to give them a a chance though - anything is better than large scale floor ripping!

@dr_dongle Living as we do on a hill, one side of our property has a deeper void than nearer the party wall. I think it is probably 400-500 mm. near the party wall and a full metre on the side where they created a trap. In our instance, the chaps were eager to donn a ‘spacesuit’ and go intrepidly down the ‘ole and carry out the spraying ‘mandraulically’ so the robot was able to have a rest. We were shown the robot buggy complete with caterpillar tracks and very powerful traction to cope with builder’s err … indiscretions etc. and we were provided with before and after photos too. We also have a ‘sustificate’ for the work should we ever need to justify the quality of the work to anyone. Regards, Toodles.

I was interested after reading this but they can only do this with wooden floor boards if you have chipboard it’s not suitable.

@irmartini Oh! That’s a great shame.😳 Toodles.

@editor Hi - An update to my earlier introduction

Our hybrid ASHP / Oil boiler system went in in April and has been doing its thing without major issues since then though the real test will come in when we get into winter. You can see the layout and schematic in the attachments - either heat source can feed the buffer tank which then delivers hot water to the radiators, alternatively the ASHP can heat the hot water cylinder directly (though not the oil boiler, a condition of the grant).

The little yellow box monitors temperature sensors t1-t7 on the pipes and outdoors (I have other thermometers around the house and nearby) and reports to HA. I will be installing power monitors on the two pumps then I'll be able to see exactly what the system is up to. I've already discovered a couple of oddities and discovered that the Vaillant reference thermometer is not in the best place.

All pumps are normally under the control of the ASHP controller but I have installed a changeover switch which lets me fire up the oil boiler and pump manually - normally the boiler would only come on when the outside temperature goes below the bivalent point.

Moving from oil to ASHP as our primary heat source lifted the house from EPC energy class 'E' to class 'C' which is a good start and  rebates from the heat pump tariff are coming through as hoped-for.