Posted: 07/11/2015 at 13:31
Sounds like a Group 2. Cut off any old flowers now then leave it until spring. In late Feb to mid March, follow each shoot back from the tip and cut to just above a pair of strong healthy-looking buds. If it is outgrowing it's space you can cut further back but always to a pair of buds. The further you cut back the fewer flowers you'll get in spring but will probably get more during the later flowering.
If it's a one or two year old clematis then the best thing to do is cut it right back hard to about a foot tall in spring, again to just above a pair of healthy buds. This will produce a much, much better and stronger plant in the future. You will get few flowers the first year after doing this but it is totally worth it.