I don't really understand how my system works as in previous houses I've only had standalone combi boilers. I'm in a new house with a single hot water tank that somehow seems to be connected to the heating too. I guess that there's a heat exchanger in there and that the water from the heating system is diverted though the water tank when it needs warmed up?

Anyhow there is a waterpump on what I'm guessing is the return to the boiler which at some point in the recent past has let out a tremendous gush of brown water (I'm guessing water+fernox) however there is now no sign of a leak and the system still seems to be working fine. Doo I need to worry?

gush as in a bucket full or gush as in an eggcup full?

like this?

there's a couple of bleed valves near the pump/boiler on my system – could be someone got a bit overenthusiastic with those or one's leaking. You can normally tell with copper pipe's where there's a leak as you get corrosion around the joint

That looks like my system, seems to be a tremendous waste of tanks etc. I'd say the leak must have been a good mug-full, maybe up to a pint. It looks like it came out from between the flange faces on the pump body, it had enough pressure behind it to spray out horizontally, but there isn't any sign of a leak now. I'll have to check the system pressure and bleed the radiators in a a couple of days when I swap the bathroom radiators out for towel rails.

There is normally a screwdriver slot in the front face of the pump which lets you let air out as pumps are water lubricated and air in them causes excessive wear. If you put fernox DS40 in the system, you get loads of CO2 from the dissolved limescale and it finds all sorts of ways out – normally as a rust staining foam.

turned out it had been leaking out of the stem on the motorised valve, I drained the system, opened up the valve and greased the packing rings with grease in a true STW fashion. I'ts been an interesting day with buckets of fernox, heated towel rails, no ptfe tape missing saws and permenantlly stained hands.