Hello Gem, first I would suggest you ensure that the mother duck has left with the other ducklings and hasn't returned. If the mother has definitely disappeared with the other chicks then you may need to find a professional rescue centre to take in the duckling as it will need specialist care especially if it is very young. Don't be tempted to take it to the nearest body of water where there are ducks in order to release it back to the wild as other female ducks may attack the duckling as they don't welcome orphans from another parent. Without knowing where the mother of this duckling is you wouldn't be able to release it as it won't be able to look after itself and needs its mother to protect and nuture it. Therefore, I would try find a wildlife centre to take it if you are certain the mother duck is not nearby and likely to return. Try this link to find a local rescue centre. You could also try phoning them to ask for advice on how to care for the duckling prior to handing it over to the professionals. Bear in mind that at this time of year many rescue centres are overun with orphans, casualties, etc... but I'm sure they can give you sound advice on what to do. Good luck.
x post with Catlady
post edit; more info although not necessary a UK website but it gives general guidance for ducklings which may help HERE
Hello Hazy - just thought I’d let you know the little duckling was fine overnight...I spoke to someone at the waterfowl sanctuary near me first thing this morning and we’re taking Ducko over there in a couple of hours. He’s currently having a nap on the tea cosy! I bought him some chick crumb and he’s been eating peas as well. Thank you so much for all your help and advice!
Just catching up again and what a lovely update Gem and so pleased to hear the duckling is doing well and that you were taking it to the waterfowl sanctuary this afternoon. Once again, well done for all your efforts, it's good to have a happy outcome. Lovely photo of Ducko and its tea cosy, mallard ducklings are so beautiful !!