What boiler is it mate, looks like a glowworm hideaway? But the square inspection hatch is throwing me off. Chances are its a floor standing open flues boiler. Could be a magnitude if things. Lack of air flow. Dust/dirt buildup. But get a gas engineer over. They will see you right. Just be aware, if the engineer is inexperienced and they see a FS open flue. They may just condemn it out of that reason alone. But if you get an old boy over or someone atleast whos worked with an old boy. They will more then likely find the cause. I still got an old ideal mexicos going for 1989 and quite a few older units going from the late 70’s with enough tlc you can keep them going. Parts are getting harder to source. But not impossible