I doubt you've got a leak Banzi,your problem sounds like a classic case of the expansion vessel losing it's charge (pressure)'
I have a Combi,and this used to happen regularly,basically the system is sealed,so when either the CH gets warm/or your using the hot water,the water heats up and expands,but has nowhere to go to,hence the expansion vessel.
Imagine a football is the expansion vessel,the football is divided into two sections by a membrane,one half is for the water to expand into,the other half is pressurised (charge).
As the water in the system expands through heat,the membrane in the 'football' expands and contracts and is supported by the pocket of air on the other side of the membrane.
As that charge/pressure is no longer present,the water now has nowhere to go, so will be discharged out through a blow off valve,which normally goes out of a pipe to the outside of the house.
The above is why you're constantly topping up the system.
So,the solution is to either charge the system yourself, or get a CH engineer to do it.
It shouldn't take an engineer/plumber more than 15 mins max to sort,so shouldn't be too expensive.