This description is not ringing any bells for me, any chance of a photograph?
Just thought maybe its Hypericum 'Hidcote', but usually that flowers earlier than August - here in London, it can start as early as late May.

Thanks so much for this - I found a picture of the flower of Hypericum Hidcote, but it could quite easily have been a picture of a buttercup ar a marsh marigold since the leaves were neither shown nor described, and remarking what the flowers were like was not my first priority when I moved! There are no obvious signs of flowers yet (but everything is a bit late this year), and it was in flower when I moved in in mid-August and carried on for quite a while.

If it is this plant, when can I prune it, and how hard? It has no obvious new growth (like paler shoots) and last autumn I did not know until Christmas that it would not drop its leaves. So I did not know if it was resting or carrying on growing. Never used to be any kind of gardener, but this small garden seems a bit more manageable, and I could do with a bit of color rather than all the just green things that are in it at the moment. So I am starting to learn (a bit late in life!!)

a general rule of thumb is to prune any plant directly after the flowers have died.

Thanks for responding - I have done some research and found pictures of the bush with leaves, which look just like mine, so this will be one of my major jobs in the Autumn. Though last time I hard-pruned a bush (a mallow) we had one of those exceptionally cold winters, and I lost it. So I will be pruning with my fingers crossed!

Actually, if it is Hypericum hidcote, you can do it right now if you want to - it will just flower a bit later. Usual procedure with this one is to hack it right back in early April, otherwise it tends to get too big for its boots by the end of the season. It's semi evergreen in colder situations.

Thank you so much - I wil try to get it done in the next week if it's not too wet. And I will have a go at it earlier next year!
Thanks again. JC