Spring, as growth begins, but don't just take the top off where the fence ends - cut back to side shoots/branches.

Also try to keep the top narrower than the bottom. That will help keep it bushy all the way down.

Once you have it the height you want it is easy enough to snip off wayward growing twigs. My Pyracantha are all well established and I have been cutting back bits that are likely to catch someone passing by. I did grow them from cuttings and kept chopping to keep the stems short. It grows quickly here. I have not needed to tie them in as they are self supporting. I planted 2' high plants from a nursery for my daughter along an ugly concrete block wall. It is outside the garden alongside a well used wide footpath. I tied that in as an espalier and it grew so well I was having difficulty keeping it in check. Fortunately I found a jobbing gardener who is doing a great job of keeping it trim. The villagers were keen to point out that I was cultivating ground not owned by the family but now there is a colourful alternative in place the village council discussed planting more around the village. Good luck with your hedge

Thank you all for your advice, I will wait until spring. Bamboo thanks for the pruning advice. They started off quite well but I realised that they were planted in a wind corridor which I have since blocked, so hopefully with a trim next spring they will bush out a bit more.

As Scotsgran suggests, if they are not too mature then why not grow as an espalier or fan and tie them in against the fence?

If there is some reason why you can't fasten the espalier to the fence try using canes. I have done this very successfully. You stick the cane upright in front of the branches that need fastening back so that they lie back where you want them. As the shrub matures you can use successive branches that are set in position to weave in canes that won't need securing in the ground. Then after a few years the framework is established and all you have to do is cut off the forward facing shoots.

Very important that you cut away any branch or shoot growing between the back of the plant and the fence. These will only get stronger and push the shrub outwards. It might look okay at first but will be a devil to sort out later.

Tht never happened with the two I did -they moved round to get the light.