Confusingly, some kinds of Eucalyptus are colloquially known as "ash" in Australia, and "mountain ash" (with no further modifier) refers to Eucalyptus regnans. See Eucalyptus common name to botanical name cross-reference But it is not commonly grown in Britain. It is the largest Euc of all, and specimens logged in the 19th century may have been even taller than the tallest Californian redwoods. Even more confusingly, the Australians call several other kinds of native plant "ash" too, like the "blueberry ash" Elaeocarpus reticulatus. I guess they were very short of names to give the very diverse plants they found there. But I don't think this is E regnans, leaf wrong, rarely grown here.

I don't think, from the leaf shape, that it is E gunni (cider gum), either, though from frequency of cultivation it is always the first guess, probably more gunnii than the rest put together. I think it is more likely, among the commoner ones grown here, to be perriniana, nipophila or pauciflora. And I expose myself as a bit of a Euc duffer that I can't distinguish those by leaf alone. Given the large size and market availability, probably more likely the first. But identifying eucs is very difficult, even if you are less of a duffer than me. There is one euc I have been going past twice a day for the last 10 years, and I still don't know what it is - certainly one of the less commonly grown ones. Because on the one hand there are so many species that are so similar to one another in several details, and on the other hand individual species are so variable from specimen to specimen. Also many species have a juvenile leaf form that is quite different from the mature form. There are over a 100 species of Eucs that can be grown in this country, perhaps even twice that, so our lack of imagination in growing so many ill-placed E gunni - they look so pretty when little but soon look out of place where people choose to put them - when there are so many more appropriate trees that look much better when mature in the kind of garden locations we have available.

You will find that, unlike a leyland cypress, cutting down a Euc doesn't kill it. Rather, like a native ash, it encourages it to grow ennumerable new shoots. You may need chemicals, or persistence, to kill it.