Is it a bit late to prune Berberis atropurpureum?

They make their flower buds the year before they actually flower, so it is important to allow the flowering wood as much time as possible to produce suitable branches for flowering the next year. If you prune too late in the year, then they will not have time to do this.
Too early and youll lose the flowering wood
Right after flowering prune out all of the flowered stems - pruning well back into the shrub. At least one third of the newly flowered stem can be cut out. It will then produce vigorous new stems to grow through the summer - ready for flowering the following season.

During this pruning, you can also cut out some of the older stems each year - by pruning right down to ground level. Around one third of the old stems should be pruned down in this manner.

As its atropurpureum, you may not be too worried about flowers - if you prune it hard now, or this autumn, you will lose the flowers, but only for a season, and this plant is grown more for its colourful leaves anyway.

thanks to you both for prompt answers. I think I will cut it back tomorrow, as you say I may lose next year's flowers but I think it will benefit long term.

Bamboo is right - atropurpureum is all about the foliage. B. darwinii for example is one for flowers.

Berberis are a very strong and resilient shrubs and you can pretty much do what you want whenever. BUT they do need light and space to grow their best.

Cats will give them a wide berth.....