Maximum permitted flow temperatures?

[sharpener]

4 hours ago, ReedRichards said:

 

Any scope to use a water source heat pump?  

 

No, as it is tidal the water does not even come up to our property boundary for 12 hours in every 24, not at all at neap tides.

 

There is also an aquifer which was discovered when we had the underground rainwater tank installed. Tapping into that would theoretically give a good supply of heat at a steady ground temp say 11C but it is 25m away at the other end of the house from where the HP has to go.