I don't know Cefn Mably, but I shall assume, also from your name that it is a lake in Wales.
In all still waters there are two ways of approaching your fishing at this time of the year.
(a) Using lures. If it is allowed, a booby fished deep on a fasting sinking line - a DI 7 - with a short leader, no more than 3 feet will work. Cast out, give plenty of time and retreive your booby VERY slowly giving it the odd twitch. Dam walls are excellent for such tactics.
Also you can try flies such as Vivas and Montana nymphs.
(b) The imitative way. This I prefer above all others and the best pattern of all in my opinion is the Damsel fly nymph fished on a very long leader - 18 feet-with a floating line. A Fast Glass clear intermediate is also worth having.
Again fish the fly dead slow with a figure of eight and drop retrieve. You need the wait at least 5 minutes after you have cast before you start this retrieve.
Do not neglect midge pupae on mild days. A Damsel on the point and two midge pupae on the droppers can be a deadly combination.