Posted: 27/12/2013 at 10:45
You can safely cut back half the growth now to tidy it up and reduce wind resistance. This will leave plenty of stems to produce new buds next spring even if some get clobbered by early frosts. i'm about to d this on a Little Nell taht has been torn of its supporting arch by the high winds. As it's a group 3 which regenerates from th ebase I can safely do this.
Given yours is an unknown quantity, just prune off what you need to make it manageable. What's left can be untangled from its support and leaned away from the house wall while you put in a stronger support. If you are attaching new trellis, screw it to battens fixed to the wall as this will allow air to flow and reduce the likelihood of disease.
Come the spring, feed your clematis generously with clematis food and then wait fo it to flower. You can then use the month of flowering and the colour to try and identify it and see what pruning regime it needs for the future to keep it looking good. This site has a search facility and lists over 3000 clematis so yours should be in there - http://www.clematis.hull.ac.uk/ it also has advice on pruning.