we had a similar problem and we managed to prop it up until late atumn when everything was cut back, though the roses were 'laid' back, a new arch installed and then the roses were pruned and layered into the new arch. 5 yrs on all is good.

I had a similar situation a couple of years ago. I waited until late autumn, when the roses were dormant, and cut them back to two to three feet. They have not covered the new arch yet, but they are well on the way to doing it

Many thanks, Seaburngirl and Andrewr. I'm expecting a visit from my son who has expressed the opinion that the arch may not last until November, which was when I was intending getting a new arch, and he also suggests propping it up in some way until the job can be tackled properly. When he has had another look at it I'll let you know what the decision is. Meanwhile, I'm encouraged by your reports of the roses surviving. I've already taken a few cuttings of the passion flower and they look as though they are OK after a few days. I'll try the same with the clematis, once I've disentangled enough of it to see what I'm doing!

Update:
I had a visit from a landscape gardener who is too busy to tackle the job at the moment, but suggested we choose a replacement arch which he could install in November. Since then, my son thinks he can handle the job, possibly building an arch from scratch, probably in October, which sounds good, as then if it turns out to be too big a job for him, we still have the other guy available. Meanwhile, our son has managed to prop up the existing structure, so it should last till then.
Thanks again for the comments.