On Sunday, January 26, 2014 5:41:08 PM UTC, Tom Pickles wrote:

Any ideas guys?

First off, get yourself a magnet and check all the pipes around the cold feed.

If there has been a longstanding leak, the limescale will precipitate at the point where the cold water enters the heating system, forming a solid blockage with magnetite sludge.

The standard remedy is to cut out the Tee and replace it.

You might be able to descale the Aerejec thing, but I'm told descaler is ineffective on the magnetite/scale deposits and I've never tried it.