this looks like a fungal thing. but the bark is missing so that will allow fungus in. Do you get any toadstools at the base of it in the autumn?
does it smell of mushrooms?

Hi.No toadstools on bottom and not a mushroom smell I don't think.Tree is over 20years old

do you know how it lost its bark?

Not sure. It'sbeen in my mum and dads garden for over 20 years but this is the first time the white has appeared

that does suggest a fungal infection then. it may be fine for years so just keep an eye on it.

Ok I will do.Thank you.We don't want to get rid of it as it's lovely in the garden.Thanks for your advice