Thanks for all the replies.
A few people say battens, and i think that is technically the correct thing to do. I'm curious though, suppose the bricks are level, why i couldn't just lean the sheet against the wall, drill about 30 pilot holes through the sheet to make marks on the brick, then remove the sheet and drill holes into brick and insert wall plugs/spaghetti , then reposition the sheet against the bricks and screw it straight to the wall ? Maybe some Sikaflex 11FC on the bricks too ... It's probably just as much trouble as adding battens, but i want to avoid losing the 30mm floor space as well as other issues with plumbing that would have to be spaced out 30mm as well. If i went with battens would i just anchor them to the bricks?
I can post a photo if you want