Different size tyres on front and rear

THIS.

With part worns you can inspect the inside of the tyre and all sorts before the go on your car. When you buy a second hand car which has tyres on it (as most do) you take blind faith as to the age and whathaveyou of the tyres.

Buying part worns off a reputable second hand tyre dealer/fitter and getting some up the car boot sale or off eBay are different things too.

I don't see why its blind faith. Look at the sidewalls for perishing, and bulging. You may have to get on your hands and knees to check the insides on the rears though. As for the age of the tyre date of manufacture is required to be printed on the sidewall

I think one reason why a mere tyre-fitter would bias towards having the wider tyres on the rear is that most drivers have never experienced oversteer, whereas understeer is the typical behaviour of a FWD car pushed to its limits.

If an unexperienced FWD driver gets oversteer, he/she would probably lift off, making the problem worse, rather than booting it and applying some oppo to pull the car back towards the intended path.

I understand that is indeed the general feeling in the trade - it's the same reason that most tyre fitting places will fit a new pair of tyres to the rear rather than the front.

However, for 10mm difference in width, I'm not sure I'd be too concerned either way.