There coud be several reasons why it's not looking too healthy.

Clematis cirrhosa is a large growing plant, and not too happy grown in a tub, it would be much better planted in the soil if possible.

Do you feed it at all? If it has been in a tub for three years it's roots have probably almost filled the tub and will need lots of water in this hot weather.

Also this clematis flowers in late winter/early spring and does tend to look a bit tatty at this time of year.

thanks for advice. I must just not have noticed the flowers before. Is more of a courtyard than an actual garden so I'm afraid the poor thing must stay in its tub. I do feed it but only with tomato food. Would something else be better?

You can buy proprietory "clematis food", which might be worth the expense if you just have the one plant. It will take the guesswork out of supplying it with the correct ratio of N:P:K and trace elements.

This plant is well worth growing. I have got my first flowers on it now, and last year they flowered all through the winter until about April. It must be one of the longest flowering clematis. Next spring you could try pruning back some of the stems to get it to branch out. All climbers tend to shoot up and have their flowers at the top unless you train some sideways. This will be easier to do with new growth. I know my garden is very mild, but we had frost in February, and my plant is not by a wall, but was fine. Yours is in a sheltered place so it should be OK too.

thanks to all for advice. I will persevere, feed the poor thing properly and cut some of it back next spring. Fingers crossed!

I have C. Cirrhosa 'Wisley Cream.' It is very vigorous and I do not think it would be happy in a pot for long unless well fed. It flowers best on the bits that get plenty of summer sun. It took a couple of years to really start flowering a lot.