Aga is working (back-boiler there), the immersion tank cold as is the pipe between the two.
Conclusion could be header tank but then the water on the taps is ok.....or more like an airlock - surprising as the piping is a good gauge to the piping and there's a 17 foot drop between the tanks and the Aga.
Anyone know how to unblock an airlock (run the water for 20 mins and try and force it out?) or any other ideas to cause and solution.....
If cold water comes out your hot taps then the problem is likely to be with your AGA's hot water circuit to your hot water tank. I have no experience with AGAs so can't say any more.
Thanks all - plumber to come over on Monday not withstanding trying to force the airlock out by running three baths equivalent through the system.....it sounds like an airlock to him........
That's very unusual in the primary circuit DT (28mm pipework from Aga to immersion cylinder).
Have you checked the header tank in the roof? If, for some reason, it ran out of water, then air would be introduced.
The plumber might undo one or both the connections to the Aga, and let them run to clear the airlock.
Annoyingly... these things often sort themselves out with a lot of gurgling in the system, but you can't rely on it.
Try connecting a hose between hot and cold taps, and fully turning on both, (hot first!). That should clear any airlock in the hot side. We once lived in a flat where the hot water system was constantly plagued by airlocks, and that's what my Dad used to do to clear them.
Solved by drawing about two to three bath-loads of water through the hot taps.....very quickly warmth appeared in the connecting pipe and, five hours later after a very nice lunch and visit to St Mawes, lo and behold, hot water sitting in the immersion tank, snug in its jacket.
Then the annual sheep dip.
Thanks to all for advice and ideas, as well as the comments.