You are marginally too late. Pruning should be done January/February before it leafs out.

It is a little late, mid February is the usual deadline, but as its cold, do a couple of test cuts to see how much it bleeds. Bleeding isn't usually too much of an issue on Wisteria, so if its okay, I'd cut it now anyway, especially if its got enormous and is out of bounds, but be careful you don't cut off any incipient flowers - these look like very fat buds. Some guidance in the link below re Wisteria pruning

https://www.rhs.org.uk/advice/profile?pid=242

I was at a talk on pruning last week given by a Head Gardener at an estate in Sussex. He says wisteria should only be pruned when the month starts with J - June, July or January (and he has to deal with one plastered across the front of a Georgian house so I tend to believe him)

Thank you Bathgate, Bamboo and Andrewr for your advice it's very much appreciated.

that's quite a clever way to remember it, Andrewr! It was always a January job for me when I had two clients with wisteria, for sure, but I was never quite so disciplined with my own, when I had one, and anyway, it was growing up an eight foot stout post, sort of tree like, so it got a lot of random cutting back at other times too. Seemed to work...

My old wisteria gets cut back at all sorts of times, whenever it gets too unruly. It doesn't seem to mind, but I don't offer this as advice...