On Monday, September 3, 2012 10:57:57 AM UTC+1, GB wrote:
The internal coil has probably failed; the crud is from the heating system.



I thought of that, but surely the OP would be forever topping up his

central heating system, and surely he'd have mentioned that? Assuming

it's an unvented system, ofc.

The pressure in the CH & the unvented cylinder would equalise, so there may not be a loss of pressure in normal use.
He did say; "This always happens if I depressurise and repressurise the tank."
The CH would leak into the tank when he had drained it, so this scenario sounds plausible.