I have recently lost a piggie to a stone and her sister (who will be 7 in April) had a bad sludge episode over Christmas. Shes not the best drinker, so we have been making sure that we top her up with fluids by syringing her with some bottled water (as much as she will willingly take), the additional fluids help pass a lot of the excess sludge that goes on to form stones (dont force it though).
We have also now been on potassium citrate for about 10 days (if your vet does agree to try it just be wary that my piggie had soft poops for a few hrs after each dose for the first few days so I supported with fibreplex but luckily she seems to have now adjusted to it).
She has 3 water bottles, 1 down either end and 1 in the middle - we have also recently started making up some bio lapis (dandelion flavoured probiotic/electrolytes mix) and then filtering all the bits out and put that in 1 bottle so she essentially has dandelion flavoured water and have noticed she is starting to drink a bit more so its reassuring to know that she's drinking more than she was especially when we are out at work etc.
Just a heads up with the sherwood urinary support - we have used it for years and although I still give it, we have still had stones and sludge form so dont put all your eggs in one basket!
Good luck!x