The welt foot hasn't arrived as yet so it will probably come Monday. Didn't have much time over the weekend to try much with the machine in any case as I've been busy with rebuilding the motor for the F350 project. I reassembled the polished foot checked the timing and all is well. I did some practice flat seems on scrap vinyl. Readjusted some tensions and all is well........so far. No problems with threads breaking or unravelling.
So it appears that the problem has gone. My inexperience with the machine making it perform a task it wasn't designed for (sewing welt with a plain walking foot) was the main cause.
Thanks for all your help on this one.
I'm sure I'll be back with other questions on upholstery in the future.