C macropetala needs no pruning, other than to keep it in bounds. Perle d'Azur on the other hand, should be pruned back to 2 buds from the ground by mid February every year. Obviously though, that looks to be next to impossible because of the difficulty of separating the stems, but if you know where the base of the Perle D'Azur is, you can locate it that way and sever the stems at that point, 2 buds up. On the other hand, the only reason for hard pruning Perle is because this variety of Clematis tends to have bare stems for half its length, producing its flowers towards the top of its growth, and the pruning is designed to get around that - in your case, you'd not notice the bare stems because of the smothering effect of the macropetala.
There's no easy solution here - the problem is you have a species clematis (macropetala) growing in with a hybrid clematis (Perle) - species always get much larger than the hybrids, often because they are not pruned, and if you try to prune the macropetala after it's flowered, you may end up inadvertently cutting the Perle, which you don't want to do at that time of year.

Sorry, to have gone on so long, but I'm thinking it through as I type this - I suggest, if you want to keep the macropetala in bounds, then your only recourse is to locate the base of the Perle D'Azur and cut it in early February, as described earlier. This should hopefully mean you can then distinguish its new growth from that of the other one when you want to prune macropetala after flowering.

Thank you for replying to my question. I do appreciate having all the relevant information on my clematis.They are much loved. I have over 35.
Clementine

So I'd guess you knew most of what I said anyway, lol!

Bamboo
On rereading my question I see that I have indicated that the Perle and the Maidwell are intertwined. I planted them on the same wall when I first started with clematis. I later tried to dig out the Perle but it grew again in the same spot so I left it and keep it under control.
I do, however, have a Henryi in Maidwell Hall on the outer 'wall' of the 2 secret garden 'walls.' I shall do as you said and try and locate it early in the season. Mine will be later than yours as I see you are in London. I am in British Columbia, Canada and we have snow-cover until April most years.
I have two Maidwell Hall clematis in this garden and both are like a thicket. About three years ago I pruned them and ended up with not a leaf in sight, in the middle of summer. I couldn't find what went where. I don't really know how to stop them getting more dense without using that drastic method. I had them covered with burlap at that point and kept it wet.
The Group B1 and B2 die back to ground most winters here. Last year the Group C did the same. Group A thrives.
Clementine

Perhaps two Maidwell Hall plants was a bit excessive, lol! Don't look much to start with, but a few years later...