Hi all complete newbie so advice needed. Bought a late 1800's cottage and want to underfloor heat the kitchen! its currently a soil sub structure so the current plan was dig it out and do a new floor build up as follows    50mm liquid screed or 10mm gravel concrete  100mm celotex with ufh clipped 150mm spacing 100mm concrete base slab   does that look about right? Also is it ok to use 10mm gravel concrete? My thinking on that is I work for a concrete company so