This is one that produces biennial stems, flowering on stems which grew the previous year, so its usual to cut out all flowered stems at the end of summer, leaving just the new stems which have grown that year, ready for flowering the following year. If you cut the whole thing down, you'll lose the flowering stems too - if you're not sure which ones are new stems, waiting to flower, wait for the flowering to begin and then remove the other stems. And although most euphorbias like light soil and dryish conditions, I've found this one does not like to dry out completely, and seems to do better in part shade.

That is extremely helpful and I will monitor the last two in the hope that they recover. I hadn't cut the flowers back which probably created the problem. Would hate to lose them as they are a vital part of my design!
Many thanks and for being so clear ... you must be a garden doctor!