I've been having a problem where our hot water gets
progressivly smeliier with an metallic smell - The last time it got
bad, I got around it by draining off the HW cylinder and cold water
tank, but it's starting again.

One thing I've noticed is We have a small copper cylinder
(about 10 ins high and 5 ins dia) which when the central heating
switches off deposits a small amount of water back in the cold water
tank - It's got SPARTAN Automatic Recovery Tank on the side of it (and
Bray Thermal Systems) and has an even smaller cylinder, with a screw
on top, on the side of it (10 Bar max operating pressure)

I'm wondering it this is the cause of my smelly water,
redepositing water from the CH circuit back in the cold water tank ?

Any ideas anyone ?