I'm tempted to say that you may be on the way to the answer to many a gardener's question - 'How do I kill this tree which keeps throwing up suckers in my lawn?' but I won't. Long cold spring, odd however that it hasn't come through that, so I would suspect something else. Pretty tough trees, but all things have their nemesis, so perhaps your tree has met its maker. It sounds as though it's a good size, from your description of it from a distance. How old is it? What's the soil like, clay, chalk? What's the aspect, north, south, etc?

we had one 20 yrs ago that just died. no specific reason but it was about 15yrs old so perhaps that's the reason.

That is all so interesting. Tree is around 17 yrs old and in good sunny position in London soil! I think you may be right and it has died for no reason.