Has the looks of a very bad fungal or bacterial attack. Since I am in the USA I do not know what you have available in the U.K. To knock this problem down so I will leave this to the GOY flagship members. In the meantime thank you very much for asking this question on the GOY site and while you are waiting for additional answers please explore this fantastic gardening site to see all the wonderful things it has to offer you. Best Regards!

Oh my Joe. That does look a mess doesn't it. Have you fed it with anything liquid, or could anything have splashed on to it...chemicals or anything?

Thanks Loosestrife2....the answer is no to both questions Cottagekarer, thanks for suggesting tho

Ok joe...maybe try some fugus fighter or similar then. What a pity...sorry I've not seen this on a euphorbia before.