I don't know anything about the Ribble estuary, which is why I used the Dee as an example because I've fished there a lot. But I would have thought low water was a good bet, and probably a couple of hours either side of it as well. But again, local knowledge is important.
Regarding species, I would have thought flatfish (flounders, dabs and plaice) were the main species with whiting and codling in the winter. I'm sure you'll get a run of bass as well in summer.
I'm sure a 3 hook paternoster rig would be a good start, fishing with worm baits.
But again, I stress, I don't know the Ribble estuary and so local knowledge is probably more useful. My contribution has probably been more of ageneral nature rather than specific. But I haave fished the Dee and Mersey estuarys in that part of the world and the above info would be applicable for them, so i assume for the Dee?