Boarding before tiling?

Hi, I''m about to attempt to ceramic tile my bathroom floor for the first time. I've removed the old vinyl tiles and the floor below is wooden. There is a thick layer of boarding on top of the floorboards so I was hoping to just apply PVA and then start tiling but I'm worried because the floor isnt level and the plywood is glued down, not screwed in.

I don't want to remove the current boards because I don't want to disturb the bath and sink, I just want to do the visible floor area (3 x 5 feet).

If I put another layer of plywood on top then I'm not going to be able to shut the door after tiling. Can I get away with just tiling as is and using a thick layer of adhesive to make sure the tiles are level?

At best: If the ply is good and firm, I can't really see a problem in tiling onto it. How uneven is uneven? If you just mean that it's not level, well check your average floor, If it's not too bad, you should be able to compensate where needed with the adhesive
On the subject of which, don't compromise on the quality. tell 'em what you're tiling over, and pay up for the best.

Also make sure the adhesive is the flexible type for putting over wood.

Sorry to but in again people, I would feel that i was doing wrong if I didn't correct you.

You can tile over wood (direct on tongue and groove boards with 'fastflex' which you would need to prime the floor firstly) or on a minimum of 15mm marine ply using a 2 part flexible class aa adhesive.

Both HAVE to be screwed down, the ply needing screws at 300mm centers using niggins where needed and with T+G at either end of each plank. Advisable to use those screws that don't rot (can't think of the name at the mo)

If you need any more help please reply on here with your questions and I will explain in detail for you.

PD Those would be zinc plated screws

is 15mm WBP ply really thick enough ?

I was thinking about removing my (fairly knackered) floorboards in a new en-suite area, replacing with 15mm WBP, then 10mm Marmox (or some other tile backer board), then underfloor heating cable, then tiles. Would that be ok, or should I be using 18mm WBP ?