Hello Uwe, HUGE thank you for your time & help. As it turns out I 'was' working the machine wrong, although I was doing as was told by my seller who I suspect may not have been as honest as he made out... on collection he did demonstrate the machine working & had said that he had been using it for the last 8yrs reupholstering car seats & that the machine had just been fully serviced was working perfect & ready for work... which is why I chose to buy his machine as it all sounded 'tickety boo' I think is the correct expression.
However, I myself being inexperienced & not exactly the brightest bulb in the house, I never asked him to demonstrate how to change the stitch length, he just said (during his demonstration) to change the stitch length you push that button & skipped onto telling me something else.
But, he told me the wrong button!!! & the mechanism that allows the concentric con rod that alters the feed dog travel was SEIZED SOLID so when I pushed the CORRECT button in (eventually) & turned the wheel until the pin locked into its little hole to hold the feed dog con rod still, I could not turn the wheel in neither direction to change the stitch length... I blathered it in thin machine oil & was working it free (with considerable difficulty) hoping & praying I was doing the right thing... it's now sort off freed off & is working as it should but it's still very stiff & I'm keeping it well oiled & keep adjusting it... What a learning curve.
I didn't do all this on my own tho, my lad is fairly handy at classic car restoration (hence my upholstery adventure) & he rebuilds gearboxes & engines & the likes