I have a boiler-Main 24 combi. Each day I have to top up the pressure on the loop.I've checked for leaks incl the overflow pipe outside but have no leaks. will boiler damage if it's still on.

Probably not, the Main 24 will cut power to the burner if the pressure drops below something like 0.4 to 0.6bar, if it doesn't cut out, then the overheat stat will be the next thing to operate. Result is no damage to the boiler. The boiler could also be losing pressure internally through leaking out of the heat exchanger, this would then go out the condensate pipe, so you wouldn't see it.

There is also an expansion tank either inside the boiler or separately on one of the hot water pipes. The balg (rubber bag inside the expansion tank) can split and cause the system to loose pressure. If it is an external expansion tank then you can check the pressure yourself by using an ordinary car tyre pressure check valve. There is, at the top of the tank is a valve, much the same as you will see on your car tyre. Firstly switch the boiler off and then check the pressure in the expansion tank, then fill the boiler to the correct pressure. Check the pressure in the expansion tank again. If it reads the same as the boiler, wait 30 minutes and check again. If the pressure has dropped, then the fault is most probably the expansion tank. This solution is applicable only if there are no leaks within the boiler pipes or central heating pipes.