Welcome to the forums @junglied. Don’t dismiss the ASHP yet because if the insulation is poor, you’ll need to address it irrespective of the heat source.

I’d definitely look into water source. I’ll ask the GSHPA about protected/SSSI rivers and water source coils. That would be your ideal solution. Do you own any part of the river?

Ground source will also work. Since land is limited vertical boreholes are a definite option. They’re just quite pricey but a good long term investment.

The property sounds terrific. Share some photos.

Before you say poorly insulated, I'm going to assume it needs a tonne of work. Have you considered insulated plasterboard? You can do a lot of heavy lifting using that to insulate inside which will massively reduce your heating costs. Yes it's expensive, but it's cheaper than a GSHP

Thanks - I won't dismiss anything. Especially with £5-6k grant likely to be available if an ASHP or GSHP allows me to remove the boiler. I will start with an energy efficiency audit to help inform my decision either way. I believe part of the river is within the title boundary but and the property has some kind of extraction allowance accordingly - will hopefully learn more when I have searches etc in hand from the solicitor.

I've got to say the ghastly brown storage heaters were doing a very good job at keeping the place toasty during both our viewings but I wince at the thought of what they must cost to run post April.

@batalto You're right - I'm making an assumption that I don't know to be true about the insulation. So first-order is to have an energy audit conducted to find out ground truth. I didn't know about insulated plasterboard so will give that some thought if it comes back as needing enhancements.

@junglied there is a tool in my signature if you want to just do your own rough survey. Do you know the current details? The building should have an EPC, which not ideal, should give you a starting point for insulation levels. I assume you aren't in yet?

@batalto Thanks. That will give me a good start. I'll get the full EPC from the selling agent (I just know it's an 'E' with potential to become 'D' according to the abbreviated version) then follow your link.

I see from your signature block that you use solar PV, batteries and ASHP to supply your home which is a similar size (especially because one whole floor in ours will be zoned for guest/intermittent use). Due to the fact I have no plumbing or water-filled radiators already in place I'm particularly drawn to the idea of a potentially less disruptive option to simply install appropriately sized and modern efficiency electric-smart radiators and whatever funky solutions are available to heat water efficiently with electricity - and use Solar PV / batteries to do so. I would miss out on the Boiler Upgrade Scheme because that requires a heat pump to meet air and water requirements for the home though. I'll be comparing this option against under floor for the whole place as the floors need changing - but I can see that getting prohibitively expensive even with £5k subsidy....

@junglied I would really not advise electric heaters. In the end you'll pay a fortune in bills. If you want the best system, go UFH with a heat pump or a gas boiler. Solar and batteries will help with a heat pump, but it's a waste with electric 

@batalto UFH over such a large space? That's an interesting thought and would potentially open up the BUS grant again if powered by heat pump... All carpets will be getting replaced anyway so it's something to look into for sure. I just want to make sure insulation is on point to make my home a good candidate for UFH given my wife's reliance on 'radiated' heat from a conventional rad (if that's a real thing)...

@junglied ufh is going to be far more efficient in a large space. If we had the choice we would have put it in, but our house was already done by builders when we bought it, so retrofit made no sense.

With ufh the water temperature will be around 10° less than with radiators. You'll probably be on a cop of 4 or 5. We have radiators and our winter cop has been around 3.

The larger and more open the space the less radiators make sense. If your wife really loves a single heat source, add a wood burner.

@batalto Thank you - this is music to my ears and makes perfect sense. There's already a large and effective wood burner so we can always get that going if a top-up is needed. 

The beautiful thing about this solution is that I can fit UFH first when replacing floors possibly leveraging the existing boiler for a while but then step it up in stages as funds/incentives/development allows (boiler to ASHP to solar PV etc). I presume individual room thermostats are all compatible with such a system if setup correctly? The whole downstairs will only be used when visitors arrive so I don't plan to heat that often....

@junglied, yes existing thermostats should work.