Hi Toby,
If you're after Grayling then the River Dee and the Tryweryn both hold a good head of Grayling. Bala angling (
newmaster) issue day tickets for both these waters, £7.50 i think they are. The Dee just below Bala (below the 'canal' like section) is a great bit of water, renowned for big Grayling.
About half hour drive away is another stretch of the Dee, run by Llangollen Angling (
Llangollen Angling Association - Tariffs). This is good Grayling water, and day tickets are £12. These are only the two stretches that I have fished often, but I am unsure about the fishing in between Bala and Llangollen.
When fishing for Grayling in winter it is almost inevitable you may come across the odd trout, as long as these are returned to the water a.s.a.p and you're using barbless hooks then it is considered acceptable. You will need plenty of heavy nymph flies with tungsten/brass or copper beads to get them near the river bed. A selection of small (size 12-16) nymphs in various colours should be fine. A light rod, 9ft-10ft long, 3wt-5wt line would be ideal. Fish a couple of nymphs on a cast with a longish leader of 12-14ft should do it.
I'd advise you wear at least thigh waders, but then again I couldnt possibly comment on the use of a wetsuit as I have never tried one for fishing!
There are a few Rainbow trout fisheries in the area that are open all year round, these might be worth a try also??
Hope this helps somewhat,
Huw