A dangerous design --- major unnecessary heat loss will result.
 
How about dig a trench - line it with 400mm insulation lay pipes - insulate them - insulate over them 400mm then slabs to cover it.

Sounds like good advice from Tony. You might also consider using smaller diameter pipes to reduce the volume left in them (about 40 litres for 25mm) and to reduce their surface area, making the insulation more effective. A bit more pumping effort would be the downside, but you'd get most if not all that electrical input back as heat.

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Tony, Mike7, Thanks for the replies.
 
I had thought of trying to insulate the pipe myself in the ground but I was concerned about how I was going to keep any insulation dry, as I believe keeping the water out is the key to well performing insulation. Given the price of the pre insulated pipes it may well come to something like this.
 
regarding the smaller diameter pipes I had been focusing on reducing the pumping effort by using as large a bore as paractical. Your point about the volume of water standing in the pipes is one I hadn't thought of. I'll investigate the effect on the pump size needed as if it doesn't affect this to much a redcution in bore size sounds like a great idea.

Richard , I've used the pipe you mentioned . Upnonor I presume, for the hot water we ran a 15mm PEX pipe inside the 25mm pipe to reduce the bore and standing water waisted,
Over a 20m run I think its about a bowl full before the hot comes through , so still not great, make sure you fit seperate hot/cold taps
You should be able to get the price down a bit also.
As for the UFH it'll loss heat to the ground once turned off
but if its a very low flow/return temp. your running you might well have it on all heating season to maximise efficiencies of heat producer ( such as GSHP )
 

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James, Thanks for the response, one of the suppliers is indeed Uponor so I'll certainly take your advice and work on the price! Also you've certainly goven me something to think about regarding the temperature we send the water across at for the UF heating. If only I was a heating engineer this would be so much more straight forward!!!

What about using solfex's solar pre-insulated pipe. Wouldn't that do the job just as well?

You could as suggested make up your own insulated pipe solution , as long as it was rodent proof.