Hi, welcome to GoY, have a look at the R H S advice at https://www.rhs.org.uk/advice/profile?PID=739
although here is no chemical treatment, it does give you tips on how to prevent it, and lists some cultivars which are resistant , Derek.

Try to improve air circulation and sun by pruning out unproductive branches. Water the roots, not overhead to keep the foliage dry as much as you can. Mildew thrives in moist conditions, shade and stagnant airflow.

You can always try the milk treatment; 1 or 2 parts cows milk to 9 or 8 parts water mixed in a spray. Doesn't matter if its full fat, skimmed or otherwise, so long as its cows milk. Best used at the first hint of mildew though...

You can still use the berries if its not gone too far - supposing you have the patience to scrape the mildew off... If its any help I've never had mildew on Invicta.
Several other varieties are blight resistant too - may be worth changing your variety if its a continuing problem.

Thanks for the inputs. I will do a thorough clean up and prune this year. I am reluctant to replace the stock as I have spent years training it into fans but I guess I may have. Has anyone tried Resolva ready to use bug and mildew spray? I don't want to use more chemicals if I can avoid it.

The trouble with sprays is that the mildew doesn't usually appear until the fruit has set. In bygone days we just used to use Derris - liberally!
Bamboo we only have goats milk - any idea why it has to be cows?

I don't know about goat's milk, it might also work... they say cow's milk to differentiate from nut and grain milks, because they think its something to do with the proteins in the animal milk that works. Best sprayed in bright sunlight, its more effective then, and dilution rates vary between 10-40% milk... less likely to smell if its low fat skim milk though. And I can't swear to it, but I think the mildew that gooseberries get is downy mildew, not powdery, so it may be totally ineffective anyway.

https://www.growveg.co.uk/guides/using-milk-to-prevent-powdery-mildew/