Is it the drive belt slipping on my Crown Point & how do I fix it!?

My 28 year old Crown Point Sewing machine is just starting to revolt. Type SANWA - SZA-645 FL / CI;
I have looked after it, oiled and cleaned it + using the vacuum to get out loose threads and dust!
I pulled it out yesterday to stitch up a cover for the dog bed and needed to refill a bobbin. Loosening the inner wheel on the big wheel as required, and loading everything up, it just kept spinning but no 'drive' on the bobbin! I adjusted the pressure wheel that sits against the bobbin wheel; eventually I got the bobbin refilledd after fiddling - it picked up when the bobbin was almost half full and then completed to fill ok;
I took the inner handle/wheel off checked it, checking the plate/clutch was back on correctly - still no good. Sounds like the drive belt just spins and it won't fully drive the needle through the thinnest of material. The motor sounds fine.
I can feel the belt is still attached with a screwdriver with some stretch/play on it; I got the job done, but 5 times longer than it should have.