This is the link to the RHS.

https://www.rhs.org.uk/plants/12557/perovskia-atriplicifolia/details

Prefers free draining soil so I suspect it is the clay that is the problem. dig in lots of grit/sand/humus to aid the drainage.
They flower in the late summer ealy autumn.

Thanks again SBG, I'm off to buy some grit.

Pennyfarthing, I also find them difficult. In the ground they grew straggly. Not a bit like the bushes i see on council plantings. My garden is clay under the compost too, SBG 's answer makes sense to me too.
Now I've put one in a pot with lots of grit and hoping it will do better.
The foliage is such a subtle colour shade of bluey/green/grey and that's what attracts me.

Agree Merlin, love the colours. Would really love to have a few wafting through the border, but maybe pots are the answer.

In Flagstaff, Arizona, they grow vigorously in full sun, in weathered volcanic cinders.

That's interesting, Tug!
Unfortunately, (or maybe fortunately), volcanic cinders are hard to come by here! However, it would be good to find out what it is in volcanic cinders that is so good for them! Do you mean that they grow wild? That must be quite a sight .
I have now repotted mine with lots of grit and am waiting to see if they improve.

None growing wild in Arizona--we only have sterile hybrids here (thank goodness!) Volcanic cinders are good for a lot of plants, but the point here is the very sharp drainage that they supply.

Thanks for the info, Tug.