Valliant boiler leak?

The house i moved into a year ago has an old valliant boiler (no idea of age). Although it works well, it has just started leaking/dripping. We can't seem to find the source of the leak, as its hard to get to the left hand inner part where i suspect it is coming from. There's no obvious pipe that it is leaking from, and the leak is not affecting the water pressure or temp. I read somewhere that it could be rusting and in need of replacement. I won't get cover on it because of its age and condition, and can't afford a new one...any ideas?