I saw they were Alsbury..Thank you all, although I'm in the UK and the Durvet product you mentioned does not appear to be easily available here we managed to find "poultry nutri-drops" at a nearby farming shop (even though it was 3pm on Christmas eve!).
Although I don't have the means to setup something like the paddling pool with shavings before Snowflake developed their current condition they really were doing well walking around despite their initial troubles, so I'm hoping that that, and the fact that I plan to move them out onto some grass soon (they'll be 4 weeks on Friday) should allow them to recover just fine in addition to allowing them to forage a bit.
Just before I went out to the farming shop the duck/goose grower/finisher food that I ordered also arrived so hopefully they'll both have a more appropriate diet. Thanks all, I wish there was more I could do immediately to help Snowflake walk around a bit easier but I'll have to go with what I've laid out here.
I really appreciate all your help, if you watch this space I may update you all on how they're doing. (They're Aylesbury ducks btw in case anyone is wondering (although not pure Aylesburies))

Best wishes