Wait till all the leaves have fallen off for you to see the structure. I would then prune it to the size you want, cutting to above a bud. They grew back well though you may lose flowering for a year.

I agree with Kildermorie. I cut a rather large one right back to the ground and had to wait 3 years for flowers. I'm assuming you are meaning a Hydrangea macrophylla.

Thanks for the advise...although the problem I have is not so much height as spread. The bush was in the garden when we moved in 3 years ago. It had very insignificant blooms in size and was very straggly. I pruned it hard back in late autumn, gave it a good feed in spring and sat back to await the outcome. The result is an extremely large spreading bush. The flowers are lovely, but its now swomping the garden. Unless I can reduce it's spread it's going to have to go and 'yes' Scottish, I believe it is a Macrophylla.

Waddy - I'm sure I've read on here that taking cuttings is relatively easy - perhaps you could get some going to replace this one and try to keep better check on its girth!
I will say that I'm impressed that you got flowers the following year :))

I think I might try that Scottish..Thanks. Incidentally, it was only this year it's flowered, but boy has it! The spread must be well over 6ft even though it's only about 3 1/2 high. :0

I work on the principle that you cut half the stems back hard once it's leafless, leaving the other half to flower the following year, then cut back the remaining, flowering stems the following year. That way you are never without blooms in any one year.
Yours sounds a bit of a triffid, Julia - what are you feeding it?