After the initial enthusiasm I think I might stick with the 400 and, as Martyn suggested, buy a sorted 600 if needs be. I did actually enjoy the ride back from Edinburgh to Derby on the 400, with the only problem being the idiosyncratic running for the few few miles after pulling off. Have arranged to borrow some electrical bits, ignition coil and alternator, from SteveH I will try and remedy the situation.
Cheers for everyones lively input.........so far!
BB