Thanks to all who have commented and helped, especially Rob, Mike and Terry. I now seem to have found the main problem, bizarrely, and it is working ok for now at least!
On my next unfurling the rotastay jammed so I went forward and gently tried turning the rig whilst looking up and down the tube. The top of the forestay always seemed ok but looking down at the drum i noticed that the line was unusually 'bunched' and bearing on the two S/S guide rods either side of the drum. It appeared that there was to much rope on the drum so i unfurled the rig fully and sure enough there was still turns of line on the drum.
I laboriously unwound all the rope manually and then I furled it in to find out what length i actually needed. Bizarrely, it didn't need any taking off; it all fitted fine with 2 foot left at the cockpit cleat... Scratch of head for a while. Some splinters later then i still didn't understand. But I also noticed two other things:
1) that the locking nut under the drum was not tight, which might agravate turing the drum.
2) that the jib could be more taut so I took off all the halyard rope, tensioning the halyard shackle through the drum U bolt and to tack cringle with one short rope. This I believe gave the tube some little extra stiffness, all to the good I say. It seems that my view of the slack forestay (which it still is!) was largely my interpretation of slack, as the thing works smoothly now.
NOTE: I have yet to follow Mikes excellent check on the internals - freedom of forestay lower end and any twisting as, having removed the 3 drum cap nuts I haven't been able to lift the drum cap and tube - sticking! I guess a gently tap with wood and hammer is needed but i will do this next visit as an essential safety check.
Tight forestays!
Philip