Hi Mark, welcome to West Sussex!
There is also Hove model boat club, which would probably be closer for you than Southwater? They sail on Sundays.
http://hovelagoonmyc.wikispaces.com/DickyD has mentioned Littlehampton, but the "club" here (I live in Littlehampton) only sail on Friday mornings, using the Oyster pond, which as DickyD says is right at the mouth of the river... One of the local councillors is very keen to resurrect the use of the oyster pond for more frequent model boat sailing, but sadly this seems to be a bit slow being taken up.
If you're just looking for somewhere local to sail, there is the lake at Brooklands park. This is on the A259 on the corner of Lancing. If you drive along the coast road out of Worthing, as if you're heading to Brighton, it's on your left, you can't really miss it! However, I think you'll find that it's a salt water lake... but I use it myself from time to time, and it has good "launching" facilities.... my only gripe is that the car park is run by NCP, so it's £1.20 for the first hour!
Anyway, I hope that this helps?