Re: pruning Clematis... it depends entirely on what part of the year it blooms... Spring (early) summer and fall (late) or repeat bloomers (one that bloomed heavily fairly early, but set flowers later as well).
Early bloomers flowers on last years wood and should be pruned reasonably soon after the last bloom has faded. They should be pruned vigorously so that new wood grows throughout the rest of the season in preparation for next year.
Late bloomers are some of the more aggressive growers... they don't really need pruning, but become overgrown and leggy if you don't. I'd recommend cutting them back to about 18 to 24 inches when the plant is dormant... late fall or really early in the spring.
The repeat bloomers don't require as aggressive pruning as the other two varietals. Check and see which blooming period is the most prolific and prune during the less abundant bloom period... just lightly though... actually, no more than removing spent blooms.
It's been my experience that pruning plums should be done in the fall/winter other than light pruning in spring/summer to remove cross branching for ventilation. Our habits here in the western U.S. may differ though from your in the U.K.
Best of luck!