I of course have never done it (zone) but the info I have is to cut it back to a metre immediately after main flowering period. It seems yours has already gone past that point. Perhaps others will chime in.

I had a massive cirrhosa growing on a deck railing at a previous garden. When it came time to refinish the deck and paint the house, it had to be cut back. IME, clematis are very forgiving. Despite the cutting back (in early summer), it regrew rapidly and even flowered that winter.

But this is from the land of giant clematis.....how well yours might respond, RP, I cannot predict but I have had good results hard pruning evergreen clems. I had to do a severe hack job on an armandii once upon a time and it never blinked!

Cheers, GG. I am going to sharpen the blades and get at it asap (I snuck off to the allotment to avoid looking at it today). Yeah, I think I am going to use my hedgetrimmers and chop back the whole thing in one. I find the usual garden cultivars to be a bit mimsy. (for me)..Even Rooiguchi looks in need of a little coaxing...but any of the ruffian wildlings (rehederiana, koreana, fargesii, terniflora etc. all grow with immense vigour...which is annoying since I have a minuscule garden and have generally had to limit myself to a single species at any one time.