now its got eagle F1s on 3/4 corners.. the rear left is a shit tyre ona spare wheel (which has f**k all grip) but i'd expect that under braking one rear tyer dont make much difference?
<Sensible mode on>
Jim, your car is RWD? Surely you know it's not sensible to have differing tread patterns on the axles let alone the driven wheels? Never had any issues on my vehicle running F1's. Different cars obviously will offer different handling & wear characteristics than my car.
I suggest you change the rear axle tyres to matching tread patterns and depth of tread as matter of course. If that doesn't fix the issue, at least you will be safer, and have one less thing to take into account, whilst diagnosing your problem.
<Sensible mode off>
Regards
Bill