Thanks for the replies!
I also thought for a moment Rob, it could be European Beech but it isn't, I do have another volunteer which is a Fagus sylvatica purpurea, the purple leaved variety and when comparing the two they don't match..
I think the Sweet Chestnut (Castanea sativa) is correct Leslieray, the shape of the leaves and the arrangement of the veins do match !
No idea how it ended up in my garden though..the seed is large and could not have been carried by birds and I haven't eaten nor bought those for many years..
