I've had problems with my Euphorbia Martinii this year. It's looked really unhappy, and the leaves have been brown at the ends. I have just assumed - rightly or wrongly - that it's the weather. I did cut mine back, but that was a few weeks ago. It's looked better since then - but smaller! My plant looked worse than yours does, but it didn't have the same blotches on the leaves. It's such a pity - I have a couple and they usually do really well.

Thanks for your comments Melchisedec. I'm sorry to hear your having problems with yours too!
Like you I was putting this down to the weather but it has only appeared over the last 2 weeks and it has been healthy up until then.
I think I'm going to have to bite the bullet and cut it back again. It's a bit late on I think but I'm a bit worried that this rust will spread to the new growth. If it gets too cold I'll try cover it with a fleece.
I'll keep my fingers crossed for yours and mine x

Thanks - I'll keep mine crossed as well!!