Simon,
I just checked the tides info on the Somers Sailing program. This Saturday and the Saturday 28th April are good for tide heights and times <img src="http://www.catsailor.com/forums/images/graemlins/cool.gif" alt="" />.
Saturday 21st will have a monster high tide of 3.15 m at around 4:00 pm <img src="http://www.catsailor.com/forums/images/graemlins/crazy.gif" alt="" /> so if you're not used to launching or beaching in big swells and beach breakers it may be better to choose either of the other Saturdays.
Regards,
Peter