Pruning group B, hmm - when I googled that, it came up with prune group 2, with which we're all familiar. If your clematis is one with large flowers that starts flowering in May, now is not the time to prune. You wait till spring and prune back to a pair of healthy buds near the top, not the bottom, see RHS page below

https://www.rhs.org.uk/advice/profile?PID=346

Prune group 3, those that don't start flowering till late June/July, are cut down to about 6 inches from the ground in midwinter, or by 15th February annually. In terms of letters, I imagine they must be Group C.

Check out the RHS site Clematis: pruning

https://www.rhs.org.uk/advice/profile?PID=109&cID=256

Pruning Group 1: Prune mid- to late spring, after flowering and once the risk of frost has passed
◾Pruning Group 2: Prune in February and after the first flush of flowers in early summer
◾Pruning Group 3: Prune in February

RHS site has some good advise for you

Thank you very much. I wrote a reply to these helpful posts yesterday but for some reason it wouldn't 'send'. I will put up with the tangled clumps blowing about around their supports and prune in Feb/March. I'll definitely explore the RHS site too.