Clematis don't really like their roots disturbed, Maryrose, but if you really have to do it, wait until the plant is dormant - in late October/November. Cut it back by about half.

Then prepare the new planting hole, and make it deep, with lots of compost in it. Dig out as big a rootball as you possibly can - try to get it out without any roots showing! Have a sack or a tarpaulin ready to put it on and take it to its new home. Make sure you water it well, and also during its first year. Feed it in early spring, too. You may lose next year's flowers, while it recovers.

There's no guarantee that it will survive, but I do wish you luck.

Thanks for answering my question .I will let you know if my clematis survives