I have ceramic floor tiles to put down only to find there is a very old, very stuck down vinyl tiles under twhat was already there.what adhesive do i use so that i don't have to unstick them?

Go to your local tile supplier and ask them for advise most of the adhesives that are available to day are flexable latex adhesives and can be applied on most substrates as long as its in good condition. Go to a tile supplier not a diy store as they deal with nothing but tiles.

I would agree with MG Build services on this but use 12mm WBP plywood. More expensive I know but it would provide a firmer and flatter substrate.

If you want to do a professional job, then you need to put in the same effort that we do! If the subfloor is concrete - Remove the tiles and scrape off as much adhesive as possible before priming the floor with an acrylic primer before laying the tiles with a good quality cement based adhesive - Ball/Weber. (Wickes sell a long handled bar with a honed bottom edge for removing the tiles). If the subfloor is timber - leave the tiles on and overboard the floor with 6mm wbp plywood screwed (not nailed) at 150mm centres, then tile on top with a flexible adhesive.