I used Unibond bathroom and shower sealant from the local shed. Expensive at 8 quid a go but it's so bloody waterproof it's v.difficult to smooth it after application 'cos it sticks to
*everything*. Flexible too, the only thing that broke it in our case was for some reason the shower tray managed to drop a few mm (don't ask why, I dunno yet!) and the stuff stretched and broke the grout on the surrounding tiles resulting in much leakage.....The moral to this tale is if yer tray doesn't drop this stuff is good! IMO obviously.
cheers
witchy/binarydinosaurs