Thanks hasta for the last tip, it's one I'll bear in mind in future.
I started using the UK420 forum (no offence guys) so haven't been back here recently, so I thought it'd be courteous to let you know how it went on. I'll attach a couple of pics and you can see for yourselfs, but basically, whilst all the fans leaves browned and died, I still got some very pleasant bud!
My next lot have started flowering (although 3 had to go outside as they started hermying - I think this was due to going into strongly fertilised soil too soon as they started growing double branches straight after they were potted in it) and they're starting to show the same symptoms. I've given them ferts and Mg so I don't think this is the problem. I think therefore that hasta was on the right track.
As he says though, it comes down to practice and patience so I'll keep plugging away until one day I get it susses!