Posted: 26/10/2014 at 16:35
Judging by the colour of the petals and the anthers plus the foliage, I would say it's a Nelly Moser, one of the large flowered hybrids in Group 2 for pruning.
This means it would usually be given a light prune and dead-heading in late June once the first flush of flowers finishes and it will then flower again in late summer, assuming it is healthy and has plenty of nourishment. However I grow mine as a group 3 as the heavy frosts here cut it back every winter. This involves cutting all the stems back to about 9" above ground or to the lowest pair of buds.
Yours looks as though its been there a long time and isn't used to producing fresh new stems every year so maybe such a hard prune is not a good plan.
Can you lift it off its current supports and lie it on the ground while you paint the wall and fix new trellis? Then all you have to do is lift it back up and ie it in and prune accordingly next June. If not, try cutting the main stems back to one or two smaller shoot nodes and then paint the wall and erect the trellis.
Whichever you do it will need feeding to compensate. In early spring it needs a generous dollop of proprietary clematis food which will act as a slow release feed. Give it a weekly dose of liquid rose or tomato food from early spring to late June. I mid to late autumn, give a generous mulch of well rotted manure and/or garden compost around the base as this will help retain moisture, protect the roots and feed the beneficial micro-organisms that help roots take up food and water from the soil.
Make sure your new trellis panels are attached to battens fixed on the wall as this allows air to circulate and reduces problems like mildew. If you hinge them along the bottom, it also makes future maintenance and painting much easier as you can simply fold them down whle you paint without disturbing the plant too much..