Well according to Google Maps, Bristol is just under 2.hrs away from me, so quite a way away, though not as bad as I thought it was.. I know the farm we got our ducks from has a ton of ducklings for sale right now, but that's quite a way away from you. I'm sure if you search around you can find somewhere more local that sells ducks
With regards to food, I use Heygates/Farmgate depending what place I get it from. If it's three weeks old I would still have it on chick crumbs. For all my ducks I had them on chick crumbs from 0-6 weeks, growers pellets from 6-16 weeks and then layers pellets from 16 weeks onwards, but its flexible by a week or two probably.

With regards to food, I use Heygates/Farmgate depending what place I get it from. If it's three weeks old I would still have it on chick crumbs. For all my ducks I had them on chick crumbs from 0-6 weeks, growers pellets from 6-16 weeks and then layers pellets from 16 weeks onwards, but its flexible by a week or two probably.