Thank you all for your replies!
I've now put a hanging bucket so all ewes can drink but baby can't. I don't mind her having a sip but she will guzzle for a while.
I checked mum for milk and she has some, baby is still suckling from her and not bleating or looking elsewhere for food.
I originally thought it was lack of milk but when she was quite poorly I couldn't get her to take bottle of milk but she was still trying to drink water, that's what puzzled me.
I don't mind her having a sip as it's good she can drink ( I know some sheep are reluctant, especially from bucket) but whenever she sees the bucket she's desperately trying to get a drink.
I haven't let her have water all day yesterday ( all ewes have had access) and she's still active and suckling and not looking for anything else.
I spoke to my vet who said maybe putting electrolytes in it would be a good idea but it's not common that they do it, I couldn't find any information on it at all.
My uncle has been a sheep farmer for 60+ years ( he's 90 odd and still going) and he's never encountered it.
How will I know if ewe just doesn't have enough milk? There's definitely milk coming out but is there a tell tale sign she's not producing enough? Or is it a habit?
When I first got her to suckle mum at three days old, mum had loads of milk and she still wanted water then.
Can I increase milk production by feeding more? Or am I worrying about nothing and should let her be with access to water?
Thanks again