It does say on the
screwfix website for plyboard overlays but not on the tub. Is this ok for plyboard ?
http://www.screwfix.com/prods/79885...-Grout/No-Nonsense-Tile-Adhesive-15kg[/QUOTE]
I only ever buy quality branded adhesive & grout products, especially for use on suspended floors; you tend to get what you pay for with this stuff. It may be OK, I've no idea but, personally, I would not risk saving a few quid on a failure & being back to square one as it works out a whole lot more expensive in the long run.
It’s described as a polymer enhanced, cement-based adhesive but that in itself may not be enough. The website says OK for plywood floors so presumable it's highly polymer modified &, in theory a least, should be suitable. The risk of using budget adhesive was yours to take & it may be OK but maybe it won't; as I said I have no experience of using it.
Also I have got a bag of unibond grout. Do I need a flexible grout for plyboard ?
It’s absolutely essential you use flexible grout & get something of decent quality; the one you mention does not have a particularly good reputation & I doubt you will find any pro tillers using it.