What you have there could well be a CATS WISKAS (Cycle All Terrain System With Integrated Suspension Keeping Alignment Static), based on the seat stays.
Take a look at the bottom bracket shell and you will find the frame number, which should start LL.
Ringo has just got his hands on a 94 model and I have just finished spending far too much money on a 92.
Take look in the British Steel thread for a lot more detail, but Dave was certificated by Reynolds to use 753, so there is every reason to think yours is an original.
Whoever sold you that for £70 did you a big favour, and as far as my limited research goes, you have a very rare frame there, which, incidentally, rides like a flat out, banzai, nutter b'stard of a bike
Get some pictures up in the British Steel thread please, 'the boys' would love to see another discovery.
Just saw the bit about the chain stays. I have just had both stays replaced on my CATS, as they were both rusted and cracked. If you don't fancy the expense, I'm sure there are members who will quite happily donate a kidney to own that bike.
"Try not to punch any clowns"