the Montana types are very vigorous and would struggle in a couple of years. But if you don't have ground space and you really want one it will be your only option. It will need lots of support to climb too.

Place some drainage holes in the tub if they are not there already. The first layer on the bottom should be a couple inches of activated charcoal or a layer of broken clay pots. If this is not done the plant roots will burn from sulfur in the stagnant water collecting on the bottom with nowhere to drain out.

Thank you both for this advice. I would rather put it in the ground but just can't figure where! I understood Mayleen was less vigorous than other montanas. Is this not the case?

Why not go for one of the smaller flowered non montana ones? Save yourself a lot of bother later.

Mayleen maybe slightly smaller, but not so's you'd notice, here's the RHS link that gives height and spread

https://www.rhs.org.uk/plants/210263/clematis-mayleen-(m)/details

Good idea, Stera but I've already been given the plant!
Thanks Bamboo. Only a tiddly little thing then!
I might have to give up the pot idea and just let it compete with the ivy on the old ramshackle shed down the garden.

Lol, that certainly does make a difference! I moved one once but it died.