If my memory serves me correct the CAA scheduled performance in the AFM for the PA28-140 for take off is flaps up. However the FAA manual also shows figures for take off with Flaps 25.
That said take off with flaps 10 degrees in the PA28 is quite a nice compromise where runway performance considerations are not a factor.
Dare I say it but of course aircraft can fly overweight and indeed on ferry flights across the pond in light a/c there is often dispensation given to fly 10% overweight subject to certain conditions. One would only contemplate this off a very long metalled runway (ie not grass) etc.
Sorry to hear our fellow aviators came to grief at Sandown. Remember the swiss cheese model - you get the accident when all the holes line up.