Hi Steve,
I tiled my Bathroom a couple of weeks ago over floorboards. I came on this site and asked the same questions and received very good advise.
I removed a couple of dodgy boards first and checked beneath the boards for pipes and cable, noting where they were sited and marking the floorboards accordingly. I used a mirror and shone a torchlight onto it to view under the boards, very important if you don't want to spend time plumbing and wiring!
Smoothed the boards with a plane to ensure there was no raised areas then put screws adjacent to every nail, time consuming but finnished with good solid floor.
The Tiles used were ceramic 33cm and Bal Fastflex. I also put two neat coats of BAL APD primer down prior to laying. As the area being tiled was only 4.5 sq mtrs I dried layed them using my 3mm spacers and measured all the cuts which i subsequently cut using my new Wet Tile Cutter.
A lot of guys where sceptical on the forum of using Fastflex as it cures in under an hour. I mixed a third of the adhesive at a time and layed the tiles to cover this area. I had wet and dry cloths available and cleaned out the Grout grooves as I went.
The finnished job looks great and i have only a 5mm step up into the bathroom which is blended nicely with a purpose made joining strip.
I have today put a couple of photos in the Gallery and you can see them there if you want.
Good luck with your project!
Petegft