If the crack is just a surface one, in the bark only, I wouldn't worry about it. If its actually a fissure penetrating deep into the tree, call a tree surgeon to check it in case the tree's about to split in two and come crashing down.

Thank you so much for your reply.
Obviously we were thinking that we could have Bacterial Canker in the tree which was pollarded a couple of years ago. We will take another look at the fissure and if it is penetrating into the tree, will ask the tree surgeon to come out and have a look at it.. It is so beautiful and we would be very upset to lose it.
(so would the birds!)

I'm curious as to the variety of the tree though - Eucalyptus gunnii has bark which peels in large, papery scrolls, and I see no sign of this on yours, despite your use of the word 'mature'. It doesn't look like canker - there's no sign of oozing anywhere, the crack looks totally dry.

Yes it is totally dry. The tree is about 27feet tall now and we were told it is a Eucalyptus. We have only lived here for two years, maybe the bark is supposed to crack and we just haven't noticed in the past!

Well it might be that its now old enough to start cracking its bark and dropping it all over the place - certainly appears to be a Eucalyptus from the leaves, but you don't usually see big cracks like that in them.

And the peeling kinds of Eucs normally start much smaller than that, too. It may be frost damage--I would expect more oozing from bacterial canker.

Thank you for your reply Tugbrethil. I am now wondering whether it could be severe frost damage after the very cold weather we had in in December. We also get Westerly winds from the Bristol Channel. Will keep my fingers crossed that this lovely tree recovers.