KK writes
Can someone advise me on a fungicide for edible plants (courgettes). My
plants suffer badly from powdery mildew (or at least they did last year).
All the spray fungicides in the shops seem to be systemic & cannot be used
on vegetables etc.

Powdery mildew seems to be more of a problem when the plants are under
water stress. Try watering more generously and more regularly and see if
that cures the problem before resorting to chemical means. Also consider
whether you really need to eradicate it - is it significantly reducing
yields? If not, removing badly affected leaves may keep some level of
control on the problem.
--
Kay