I have found over the years that the period between mid October and the middle of December used to produce the most numerous bags, and then from January to mid February the best time for a lunker.
However the fish have to be there and this year the fishing in the North East has been poor. Was talking to a match angler last night who said he is even starting to become disillusioned with the fishing.
The past few weeks have shown a slight improvement on the past two years for january(probably due to the fact that there have been a few decent seas), but the fish are patchy and it's being in the right place at the right time. I know lads who have gone down and had 5 fish one night, gone to the same area the next night and the cod are gone.
As for the end of the season, it really depends who you are as many anglers still fish the kelp beds for red cod during the summer months. The Alnwcik Sea Angling club always finish between the middle of Feb and the beginning of March as it really dies off then.