Spotting

Thanks so much Meik! Looking at the info on Ganoderma, do you think it would be a good idea for me to warn my Mum (this is a tree in her garden) that this fungus will eventually mean the death of this tree?

Yes, it is a tree fungus, my first guess would be Ganoderma adspersum. It often occurs on deciduous trees like Fagus sylvatica and can be found in oceanic climate. It usually grows in wounds near the bottom of the trunk. All of this would fit to your spotting. However, I'm not an expert on this topic but maybe this might help you a bit.