Ok, front tyres replaced this afternoon and got some feedback from the fitter too as I told him beforehand what problems i'm having. These are his observations:
1. one of the tyres was indeed out of shape and he showed me it too
2. both tyres had different production dates, one was two years old, the other was three years old.
3. the passenger side front wheel has a 'slight' buckle in it, which has required a lot more weight to balance.
Now i haven't driven the car yet to see what difference it makes, but i'm already glad i had the tyres changed from the observations. Now all 4 tyres are brand new, trying not to think about the cost (£1000)
The fitter has advised to give the car a go and see how the brakes and steering feel. If still juddering then he said I should just replace that one alloy. I've seen a good one with no buckles on eBay.
I then asked him whether the slightly buckled wheel may cause the new tyre any damage, he said it shouldn't. Do I need to be concerned?
I'm now pretty sure that one alloy is buckled because a few months back I had the fronts balanced and they advised of the 'very slight' buckle in that one wheel.