Thanks for all the help, I think I cracked it (not literally). For anyone else trying to do the same this seems to have worked:
Used 10-12mm flat wood bit (buy cheap one you can chuck when done) and pre-drilled countersinks about 4mm deep into the Aquapanel where I later fixed. (Even when screwing into timber, I found I had to countersink a bit just to get the proper Aquapanel screws to sit flush).
Used normal (Wickes) plasterboard adhesive to get the Aquapanel on wall. I did big wide strips at about 250mm centres instead of trying to dot, and the thickness was about 6-7mm thick. This felt pretty solid to be honest, but maybe it would have failed over time.
Drill through the countersinks, adhesive and into the wall (quite a bit deeper than your fixings I found). If any sound hollow (i.e. you missed the adhesive) then don't use.
Use some small (I used No. 6 x 60mm) hammer fixings, although screw rather than hammer them in. I went for stainless steel ones in case of corrosion, but don't really know if that's really a worry and they are quite pricey.
A bit laborious but it seems to have worked. Thanks again for all the advice. I'll get this bathroom finished one day...
Used 10-12mm flat wood bit (buy cheap one you can chuck when done) and pre-drilled countersinks about 4mm deep into the Aquapanel where I later fixed. (Even when screwing into timber, I found I had to countersink a bit just to get the proper Aquapanel screws to sit flush).
Used normal (Wickes) plasterboard adhesive to get the Aquapanel on wall. I did big wide strips at about 250mm centres instead of trying to dot, and the thickness was about 6-7mm thick. This felt pretty solid to be honest, but maybe it would have failed over time.
Drill through the countersinks, adhesive and into the wall (quite a bit deeper than your fixings I found). If any sound hollow (i.e. you missed the adhesive) then don't use.
Use some small (I used No. 6 x 60mm) hammer fixings, although screw rather than hammer them in. I went for stainless steel ones in case of corrosion, but don't really know if that's really a worry and they are quite pricey.
A bit laborious but it seems to have worked. Thanks again for all the advice. I'll get this bathroom finished one day...