Clematis - prune hard back now?

I have a clematis that has just finished flowering. It was in situ from when I moved in to my house and it stayed put when the garden was "done", but its now looking a bit of a mess and needs some proper suport - previous occupants simply left the plant tied to the original two foot canes it came in and provided no additional support or trellis. It is only by sheer determination it has managed to reach the top of the fence by using a scraggy winter Jasmine that was next to it, but after that was removed its all collapsing on itself. OK to hard prune back now to allow me to install a trellis or should I wait will next Spring before new growth happens?

You have several options really. Since its flowering in May/ June, its almost certainly Group 2. Large flowered hybrids that flower twice a year, right now on last year's growth and a second flush of smaller/single flowers in late summer on any new growth this year.

Usually, these Clematis require little pruning but yours needs better support so cutting it back will not kill it per se, it will just have some extra growing to do and there will be loss of flower. If you prune now, you will lose the second flush of flowers in late summer + next years, since buds are made on new growth this year.

 If you prune late summer, you will only lose its big flowers for May/June, but it will undoubtedly send up new shoots in spring and may pop out a few small blooms by the end of next summer.

Don't forget to feed it in spring.

Oh damn this lack of edit...another thought, it could be Group 1 Clematis as some of these sort of cross over from early spring flowering, but produce a profusion of smaller flowers as opposed to very large ones.

If it sounds like Group 1, you can prune now and it may still have time to grow some new shoots to flower in spring next year.  You might want to feed this one after your prune though, as they tend to be quite vigorous.

Can you not just detach it from whatever i sholding it up and lay it down while you install a decent bit of trellis?

Failing that, just cut off what you need to release it but leave as much foliage as possible to carry on feeding the roots and making new buds.  Give it a tonic of liquid rose or tomato food to help it regrow.