I am not a trades person, but renovate properties and do all my own work, so no axe to grind here.
Boiler filters are notorious for producing leaks. I have one at the moment on a kitchen refit i am doing and i had to move the filter slightly to the left to get some unit in place. Now it is leaking a touch, so i need to tighten some valves up. What has possibly happened to you is that the filter has been moved slightly and so has started to leak from one of its compression fittings.
After 2 yrs without a leak you cannot blame the engineer for poor quality work.
should he have done a service ? Prob not, but would you have had it serviced anyway ? If so what is the problem.