Hi Howard,
I would agree with Darren, if you have a flavour that they find to their liking now, they will also in winter, personally my flavours are always of a spicey / savoury nature.
Regarding bait, Darrens comment on texture is probably the most important of all, most shop bought boillies are by their nature hard, with a Barbels metabolism already slowed by cold water temps, will mean an even slower process of digestion, so i would reccomend only paste as free baits, and be very conservative with it in how much you put in, that obviously means as Darren says location becomes a vital consideration, even more so than usual.
To that end i would make your freebie paste fairly soft, to ensure breakdown in the cold water leaving a flavour trail the Barbel can home in on, even soft paste in very cold temps will last many hours, but the flavour will remain on the bottom for some time leaving a trail the Barbel will follow if it has the desire to eat !
I always summer and winter mould paste around my hookbait, and you can make that a bit harder to make sure it will last several hours, and i always mount 2 baits on the hair which the paste will get jammed between the two, and last even longer.
You can boil your hookbaits as long as you need to make the firm enough to take the paste being squashed around them, after all they are never going to eat those ! but if you still wish to boil your freebies then i would boil only long enough to put a skin on them, but if the temps drop to the low 40's i really would advise using paste only, as free bait, so as to ensure that as soon as the barbel has eaten it, it's passage through their gut will start immediatly, considerably reducing that full feeling, that they will have after eating even a small amount of harder food, which would remain in the top of their gut for many hours.
So if you have a flavour that works now stick to it, and as Darren says concentrate on location, and texture of bait.
Regards
Ian.