Cadbury choc fingers, all varieties I think are 1 1/2 syns each. Usually on offer in Wilkinsons for loads cheaper than in the supermarkets.
Not strictly chocolate, but the choc flavour mini twister lollies are only 2 syns.
I've been trying for ages to buy Cadbury animal biscuits (the ones in boxes) as they are only 1 syn each but can't find them anywhere. They usually have a festive themed one around Christmas time (Santas and reindeers and the like) so i think I'll have to stock up if they bring them out again. Christmas stock usually starts appearing around September so hopefully not too long to wait...