Fungus find

I took the following picture of some fungus during the late Summer and ever since have been trying to identify it.

Tree fungus enlarged 

Tree fungus

The fungus is relatively flat on the top with a slightly swollen underbelly. It has a mottled pattern on the top surface. It bears some resemblance to Perenniporia fraxinea but it is unlikely to be this variety as it is quite rare in England. It could also be Piptoporus betulinus which is lovingly called ‘Razor Strop.’ The problem I have is identifying the plant that it is actually growing on as although it is one of the most commonest of tree fungi, razor strop grows almost entirely on the trunks of birch. These particular fungi are noted as greyish brown and smooth topped … this is the correct colour but it does have a knobble or two.