After giving it some thought I've swapped my mileage rate around so rather than claiming business miles at HMRC rates I'll now put all the fuel for my company car through expenses and pay back private miles at the HMRC rate. As prices drop I'm sure there will be times that my applied pence per litre amount is more than the actual fuel cost but I can't see the deficit ever being as high as it is now (around 30 pence per litre!) and I have the benefit of my fuel cost now being calculated at a fixed 52.7 MPG which is right on the upper limits of what is actually achievable in my car (long term average on the trip computer is normally 46 to 47 mpg).
So you have me to thank when fuel prices drop through the floor on 2nd September the day after HMRC set their next rate