Many thanks indeed, nickso & JohnD. I'll try that. So basically you're saying it's quite likely a pile of rust particles that have stuck together.
Thanks again.
Hi
I'm pretty certain the feed from my central-heating attic tank down to the boiler/rads is blocked. [Why? (1) Because when I drained the entire system, opening all rad bleed valves, the tank was still full ! (2) because having refilled through the draincock (system now running but there's...