Ian,
The only absolutely known quantity here (assuming you don't drain the tanks) is how long it took you to get to East Midlands. Even with a dipstick, you are only estimating the fuel left and your 5gph is also, presumably, an estimate.
So,
estimate how much fuel you used to get there, divide by time and that gives you a revised
estimated fuel burn. Assuming you did not do/will not do anything unusual - eg extra long taxi/hold, lengthy full power climb - that will do as a reasonable
estimate . Now, take the
estimated fuel remaining and divide by the revised
estimated fuel flow and you have your revised
estimated endurance.
Remember, though, this is a total endurance figure and does not have any reserve built in. Also, it is an
estimate 
Safe flying
Aiglon