Having got the saddle back on time to take the bike for another ride. Fixing the saddle nose certainly improved thing. every thing feels more stable now, the rear of thre saddle could do with going up a little more I think - the back of the saddle is hitting the rear mudguard some times, but I had fitted to longest pieces of studding that I had to hand. Seemed however to have another problem now, a rather nasty rattling noise from the rear end, especially on the "over-run".
Looking at the drive chain I noticed that there are "witness marks" on the chain, clearly been hitting something. However i also noticed that the chain was quite slack, maybe that's the cause, allowing the chain to move about more than it normally would.