I'm about to tile the hall of an old Victorian house. Total tile area
approx 16m2 of 8mm thick faux Victorian tiles. Proposing to put 12mm
plywood over 22mm floorboards.

Can anyone tell me the recommended fixing for the plywood to the floor
please? Are the screws supposed to be at 6", 8", 12" centres or
something else. Tight joins between the plywood sheets? Is it OK to
leave a 5-10mm gap all round at the walls to make it easier to get the
plywood into position? What screws do you recommend? I'd like to screw
straight down without having to put a separate clearance hole in the
plywood or countersink it. I don't want to screw down into the floor
joists for fear of catching the central heating pipes which have been
recessed into the top of the joists.

Finally I've been told I need flexible tile adhesive. Any
recommendations please?

Thanks