Boiler leaking but pressure is fine, what could be wrong?

I noticed my boiler was leaking (only had it installed about a year ago). I checked the pressure which was at 0 so I topped it up to the middle of the optimum pressure level. It has continuned to leak on and off every few days, normally when it is very cold outside. The pressure has stayed the same since I adjusted it about a month ago. Anyone any ideas what could be causing it? How much would cost to fix etc? It is a worcester combi boiler. Heating for water and central heating both work fine.