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.