There's a source here
http://www.express.co.uk/life-style/garden/649164/Busy-Lizzies-garden-back-better-than-before-flowers-plants
that suggests the problem has gone away, but I can't find any horticultural source saying that's the case, so not sure where they got their information from.

I've got friends who've grown Impatiens walleriana in 2016 from plugs they bought without any problem, but I've not grown any myself. Might be a case of 'suck it and see'...

They are going back into the retail market but I don't know how they licked the problem. The only good thing to come of this was that this plant was overused ad nauseam in the USA and gardeners became familiar with the many alternatives which were formerly overshadowed...:)...by Impatiens.

I grew some last year without any mildew appearing.

I grow them every year with never an issue. They bring a splash of color to my hosta display. The main reason for downy mildew is moist, warm, and stagnant air flow. This may be difficult to control in the UK with all the rain/fog you get. Keep the ground moist & the leaves dry. Clear away surrounding vegetation to improve airflow around the plant. Bright shade/dappled sun is good, early morning sun & evening sun will help repel mildew.

thanks guys

Gg