Geoff, Mark's quite right and for cold conditions I'd opt for a single maggot. I've found on my local lake that dead red maggots, surprisingly perhaps, outfish live ones. Particularly if you impart some small movement in the bait. A slight lift, an inch or two to the right or left etc.
Live maggots will shock and stretch on contact with really cold water which is why deads are best I think.The best way I've found to kill maggots is to riddle them and place in a freezer bag. Remove as much air as possible and tie off the bag. After 10 minutes or so you will see that the maggots have used the remaining air and stopped moving. THEY ARE NOT YET DEAD. Place them in a freezer overnight, take them out of the bagand then thaw them out IN WATER. They will not be stretched and remain plump and well usable as hookbait. Keep themwet to stop them drying out.Give it a go with few and see if they work for you.