"For example: John Smith" wrote in message
...

The alternative would be very thin wall radiators - if such things
exist!.

I have a single panel radiator in smaller space than yours. Total distance
from wall to "outer" edge of rad is 75mm.

That would not eat up much of your 800mm. I have much less width than
that,
and still have room to pass.

In my understanding (as a prospective buyer, not an expert), 40mm is not
enough to for a proper installation of underfloor heating.


40mm is on the edge, 50mm is OK as long as you use fibre reinforced screed.

You could proabaly get away with 40mm if you used a 100mm square steel mesh
about 10mm or so up from base of screed and used a fibre reinforced screed.

Rick