Cor, that is one of the worst infections of mussel scale insects I can remember ever seeing. You need to do something really urgently of lose the tree.
I have answered this before but I will repeat it for you. The white stuff is a wax which you have to dissolve to kill the insect. So, first wash the tree with a soap solution. Personally I would use a horticultural soft soap, but liiquid soap will do as this is so urgent. Use a scrubbing brush, not too hard bristles and scrub the bark with the soap solution. Experiment with the strength until you find out which gets rid of the things. Then I would spray with Provado if you have it. If not then wipe over the cleaned bark with methylated spirits on a cloth or parafin if nothing else. Then wash the tree with warm water. You may have to repeat this treatment a few times to finally remove them all. Not a nice job, but it is that or dig up the tree and burn it.

I've not heard of that Ob --- it looks Awful

never seen an infestation as bad as that either.

thanks for the tip. I've scrubbed all 3 of them tonight and they look so much better. Will try and get some Provado tomorrow.