That's the problem. I have the manual for the boiler but not the cylinder. It's made by Junkers but it's one of these "white boxed" units that looks like a fridge. It's all rivited together and full of foam so I can't see inside. Just the four pipes coming out plus a temperature sensor wire. There might be an expansion unit in side but I'd never know.

The only writing on it refers to replacing a magnesium anode every two years.