There are also a few run by total buzz, one in wincanton, gillingham and Taunton i think
Unfortunatly the 2 Sherborne ones are a no go for me, the May one is the same time as the Tour of Wessex i'm doing and i'll have to check but the July one is either the same day or a bit to close to another event i'm doing. Worth remembering for next year though thanks for the tip.
Take a look at the naked strength events, i did the Bath Multisport super sprint triathlon last september. It was my first attempt so i went for the shortest possible incase i hated it, infact it was brilliant and i am signed up for 2 more this year.
They are designed as real novice events, so if you make a mistake you are just helped to do things right. I saw loads of people getting on or off their bikes the wrong side of the mount/dismount line but there's no disqualification the marshall just call out to you to tell you what your doing wrong. A huge range of outfits on show from tri suits, to a lady in a one piece swimsuit which she did the whole thing in with nothing else not even a tshirt. (brave lady!) There was also a massive range of bikes, loads of mountain bikes and i'm sure i saw a basket going around the bike leg.
Well worth a look if you want to try a super sprint tri first, now i'm off to look at the Sherborne one you mention as i only live down the road and hadn't seen this advertised.
Total Buzz have a couple in Wincanton and Gilligham i think, not sure if that is west enough for you. Naked strength have events in Bath, i did the super sprint last nov at the multisport weekend and i think they have a weekend in July as well.