Thanks Miriam; I replaced the tension assembly when I took possession of the treadle about 12 months ago and this is the first time I have sewn on it since cleaning it up. The treadle has been stored down in its cabinet all this time. I have run some thick thread backwards and forwards through the discs and it has come out clean. I am perplexed why it only happens when I'm treadling rather than when I'm hand turning. I guess it will turn out to be something obvious so if anyone wants to jump on a plane and pop over tomorrow to straighten me out (and have a laugh at me!) come on over!!!!