Things have moved on slightly from the days when the only clematis you would have found in Britain was old man's beard (clematis vitalbo) the rampant climber which can still be found in our hedgerows and roadsides.
Nice now that we have a large array to choose from thanks to the arrival of other species from Italy in the 16th century, then Japan in the 19th century which gave Europeans a chance to develop larger flowered cultivars that we know and love today.
The Victorians were particular fans and it was at this time that the famous C. 'Jackmanii' was created but unfortunately the dreaded clematis wilt also appeared which meant clematis popularity declined somewhat for a while.
With so much choice now confusion can come when pruning but things can be kept simple with a split into three groups:
1 - early flowerers - producing single or clusters of flowers on shoots produced the previous season - no regular pruning is really required only light tidy up straight after spring flowering leaving remaining growth to ripen and produce next years flowers. Examples: Armandii; Forsteri & Montana
2 - midseason flowerers - May-June - these clematis produce large flowers on shoots developing from previous year’s growth and can flower again in late summer on new growth. Prune in late winter/early spring or back to a strong bud, as soon as flowering has finished. Many clematis in this group are good to use to climb up any shrubby evergreens that may have finished flowering or planted together with another climber like a golden hop, extending the season of interest and providing a support for it at the same time. Examples: Clematis ‘Jackmanii Alba & Rubra’; Clematis Liberation & C. Beauty of Worcester.
3 - the late flowerers - this group flower mid-late summer on the last 2ft of the current years growth, so if you ever see plants that are a tangled mass of growth just flowering at the very top these have not been pruned! They need hard pruning in February to the lowest pair of buds. Examples: Clematis ‘Abundance’; C. ‘Lady Betty Balfour’ & C. viticella
If your clematis suddenly keels over it could be wilt, there is little known about this fungal disease, which causes leaves to wilt and turn brown, and stalks to turn black. Unfortunately there is no chemical control available for it but you can help lessen the impact with cultural control by cutting back to healthy stems, clearing and destroying of diseased material to help prevent re infection. Clematis is a plant that prefers deep fertile soil in a moist shaded habitat so the closer you can create this environment for it the better.