is there any one on The Isle of Wight on this site that could give me some advice?

Hi Penny, I would take them off and start again, the ply board is a good idea putting tiles on plasterboard walls can cost in the long run when the water gets behind and it and the wall falls down, but it should have been fitted to the studs behind the plasterboard (using a stud detector easy to use and very cheap now) it should be screwed in place. And for keeping the tiles the same height you can use a length of wood place it over the every one or two lines/'s of tiles you have fitted and you will see any gaps between it, some pressure on it and you should push any raised tiles down to a similar height (this can be done with a long level but do not be too critical with it). If you attempt to do this yourself and it is not too hard, visit youtube "tiling" it has lots of help on doing it your self. Hope that helps somewhat. Kenton Farrell.