Yes supermarket fuel most of the time and thinking about it, the initial symptoms and recent worsening both tie in with filling up at a certain local supermarket.
Thanks, will give it a go and hope it fixes the problem!
Supermarkets often cock up diesel blends.
If you have a shell/bp/texaco etc near by I'd strongly advise issuing that instead in the price is close. An extra £1 or so each fill up will save £100s in the long run