I'm wondering if you mean Vyvyan Pennell rather than Pendleton? If you do, have you pruned it at all, and if so, when?

Thanks for correcting the error .
No,the plant has never been pruned as it was only bought last year and was in bloom when we purchased it .

Okay, how large is the container it's planted in (depth and width)?

The container is 30 cms internal diameter and 30 cms deep .The " Soil " is a mix of peat free compost with the compost the clematis came in

Transfer it to a larger pot, 2 feet deep by 12 inches wide, use John Innes No. 2 or 3 compost, you can mix it with multi purpose if you want.

Thank you for the information .i will be out later to get a larger pot .