Quick Update.
Got the alignment checked at pro-grip in Larbert - great service.
What they found is:
Rear offside - slightly toed-in.
Rear nearside - slightly toed-out.
Front in alignment, except for too much negative caster.
Also, nearside front 6mm
lower ride height than offside - no issue with broken spring (maybe the previous owner had a lardy passenger?) - or alignment.
He brought everything back into alignment - which also involved repositioning the front sub-frame slightly.
Took about 90 minutes all in including the gassing about life, the universe and everything.
Results - The suspension is crashy and still not great, but when the car skips it stays straight, so a decent result as it's improved slightly but likely to see further improvements when I ditch the Bridgestone tyres.
So, if you suspect alignment and are in central Scotland, I do recommend pro-grip (no, I didn't get a discount).
At least it's one thing eliminated.
So, on to thinking of damper changes at some point.
As a counterpoint, test drove a new mini cooper s at the weekend (wife's lease is up soon and that;s what she wants) and it was firm, but much more compliant and secure, so to me it shows that Seat haven't got the ride/handling thing sorted properly.