Assuming there's no sign of bracket fungus on the trunk anywhere, this could just be irregular water supply - that can cause longitudinal cracks in bark and it then flakes off like this. Only other thing Sycamores suffer from is Sooty Bark Disease, but you'd probably see blackening underneath the peeling bark.

Thank you very much Bamboo. Could well be that as it is growing on top of a four foot high earth bank, which is a traditional "wall" round here. There's no sign of blackening. So I won't waste any energy worrying about it!

Bark splitting has been a huge problem over recent years, down to a long period of mildish winters followed by recent harsh winters, have noted Bark splitting on many shrubs/trees since last winter, judging by the picture, has the tree suffered a would in the past, to the right of the trunk is that the rolling effect of the tree calussing over, the bark looks brittle in the centre and it looks like the inner wood is dead on this part of the trunk, would i be right in assuming the wood beneath the flaking bark is corky, if so then the part of the trunk that is alive and callussing will eventually over years envelop this, which will stay rotten and may be a problem in the future, check the wood to see if its dead and corky first.

We have only been here for 6 years, and there has been no damage to the trunk in that time. I think the cracked appearance is more the bark coming off than callousing but I will go and have a look tomorrow to check for corkiness, if I can reach it. Its not in a position where I dare put up a ladder. We will certainly be keeping an eye on it as its not in a place where we could risk it falling.