Boiler Leak Sealer

Least you know its not the boiler! Check around radiators, pipework under boiler, around any compression joints in general for any signs of pipework sweating, or pool of water forming if its not around radiators or boiler then its in pipework that isn't visible and leak sealer is the next logical step to try. if it doesn't work then a company with good thermal camera's might be the answer to try see if the heated water is dissipating in the walls or under the floor. a loss that much over a few weeks as mentioned above is not a tiny amount, so there would be evidence eventually, dampness etc

Leak sealer is a well known application for hard to detect leaks, 0.5 bar in 4 weeks would mean your system inhibitor is virtually non existent so if you do it yourself, there is no loss, but sometimes I have used leak sealer and it just does not work, usually because the escape of water is too great , check under floors, especially if someone has fitted compression isolation valves, usually the turn valve leaks at the place where you put the screwdriver (should not have valves under floor) check all the radiator valves and connections, last place is boiler(this is where you need an engineer!!!!!) In short leak sealer works only for the tiny leaks 0.5bar every 4 weeks is not tiny! You should check with the manufacturer with the warranty!

All all the previous answer. I am assuming from the description this is a heat only boiler. If so, and there is an existing indirect vented cylinder; test that there is no leak in the cylinder coil.

I would check for obvious leaks, everywhere, You might want to check that the system as enough expansion, some boiler manufacturers advise to increase the expansion by adding a external expansion vessel to the system and you might want to check with the boiler manufacturer about leak sealer as it might invalidate the boiler warranty.