Hi Steve. I was catching in early January in Cornwall last winter. Until the first cold snap. Then they were pretty much gone. They came back again in April, but it was slow to start. Hard baits doing well here from about July/August, but earlier in the year soft baits were definitely better where I am.
South Cornwall, to the west there are odd fish all year. The water temperature stays a touch higher than it is on the North so odd fish are almost always present. Hard going though.
from my experience in dorset id say the season generally starts in april, gets better and better until October, then starts to tail off until dead around xmas/ early january. Does depend a lot on the weather though. Last winter was harsh, the fish disappeared early and showed up late. There is the odd fish around all year, but i go for the freshwater species january-march. I dont bother with surface lures at the end of the season. Hope that helps.
Will concur with the previous replies on when fish are here and when they are few but particular attention should be paid to what the fish are feeding on at particular times through the year. If you can match what most bass are hitting at the moment in time at a particular location the likelier you will get hits from them. This can mean different colour patterns, hard or soft plastics lure shape and action. Knowing what the bass are taking takes looking at the stomach pouch of fish you have taken to eat if thats your bag? or spending large ammounts of time spotting what is swimming around in front of you while youre getting blanked trying to get the lure right.