On the subject of fuel i work for a ford main dealer and over the last two weeks ive had 5 vans with problems on running which have turned out to be down to fuel quality. all the customer are well known to us and all 5 use only bp, tesco and shell.
but before they had the running concerns the last place they filled up was bp. On checking out bp website as they sponsor a part of Ford i found there is a fuel call bp30 on the market which is part biofuel. so with the crap ive seen in the fuel from the petrol transits i would pretty much say its this stuff, the vans had to have a full fuel system to get it running as the injector spray patten was s*agged and fuel pressure was to weak.
As for the diesel theres just excessive water build up rusting the fuel filter blocking the tanker sender units return.