All great advice, thank you. The shower will be used maybe a few times a week, no real heavy usage at all. I think I've decided to butt the bath up to the wall at one side(nearest shower) and have a bigger gap at the other end and maybe tile the small infill(yes I meant 15mm not 15cm...doh). The reason I wanted it centered is because the taps are in the middle as opposed to the 'end' and im having a mosaic type pattern straight up the middle. My problem is I'm really indecisive and read far too much on the interweb regarding different peoples views on the adhesive side of things. As I said I've used the tubbed stuff before without any problems in years and no tanking - but that was on a regular old plastered wall. Oh - the shower is just a mixer fed from a combi-boiler with good pressure and the tiles are 9mm thick and 300 x 400mm. Thanks for all your help guys. Really appreciate it.