No, they flower perfectly well in the open ground - I've just discovered that you might get bigger flowers when the roots are restricted (mostly done for Show purposes). Need fertile, well drained soil, and not in shade. I'm assuming the ones you want to plant are hardy - quite a few varieties aren't, though in London, you might get away with it anyway.

Thanks Bamboo,
Brilliant advice, as usual!
Charlie

ps I saw convolvulus (still can't spell it) for sale at a farmers market today...

I can't spell it either, but what's important is we know what we mean, lol - which one was it? I have Convulvulus cneorum and Convulvulus sabatius

I stupidly didn't check, but they looked just like the one in your photo - which is the same as mine, I think. Over 10 years old and looking great! I might even take a photo and up-load it!
Re-potted it in April - which seems to have given it renewed vigour.
Charlie

I lost mine this winter, but luckily managed to get a replacement - last one in the garden centre!

Phew. If I had to walk out into the garden and not see my faithful convolvulus it just wouldn't be right!