No hot water

Get an electrician in to look at the plumbing.

 

I'm guessing what the plumber suggested was to connect a cold 'mains' tap to a hot tap, and then gently allow the higher cold mains water to push back the air bubble up to the water tank?

Yes, this will cause the tank upstairs to overflow (safely out the overflow pipe, I hope...?), but that's ok. It should still do the job.

Does you kitchen have a mixer tap? If so, you can use this. Get a small plastic bag and slip it over the nozzle. Tape it firmly around the nozzle stem. Open the hot tap fully and allow the bag to fill - it should then stop. Then slowly turn on the cold tap - judge how far you can open it without causing the bag to burst!

The mains pressure cold should push the hot back up the pipe via the hot cylinder and to the cold tank in t'loft (which will overflow after a while). Try and leave the tap running for a good, ooh, minute? Perhaps less will do.

Keep checking that the tank in the loft ain't really overflowing...

Close off both kitchen taps and then try a different hot tap in the house (one that had stopped working) and see if it now flows. Turn it on FULLY so that the initial fast flow will take with it any remaining air bubbles.

If that hot tap now works fine, you can undo your bag. If not, retry the kitchen but keep it running for 30 seconds longer.

(Why did the air lock reappear? That tank in the loft didn't empty again, did it? If so, check that new ballcock.)

 

Impossible for us to tell any more from this side of the screen, I fear.

Since you have tried the usual air-lock shifting ideas, that it does sound less like an air lock now and more of a full-blown blockage.

All I can suggest is to isolate sections of the pipework - the one you most suspect are blocked - and try and flush them out using mains pressure water.

If you are saying that all the hot taps having stopped flowing, then it seems pretty obvious that it ain't individual pipes going to individual taps, but instead a common pipe that supplies them all.

In which case I'd be looking (a) the cold feed from the bottom of the CWS to the bottom of the hot cylinder, (b) the gate valve you should find on that pipe chust before it enters the hot cylinder, (c) the hot water outlet at the top of the cylinder, along with the the 22mm pipe that goes from there and feeds all the hot taps.

Not sure which to try first, or how to go about it. I guess I'd do something like - shut off the CWS inlet (its ballvalve), attach hose to drain cock as it enters the bottom of the hot cylinder, drain the whole cylinder and CWS down, and then monitor what actually happens during this process.

What should happen is that the whole hot cylinder and CWS should empty fully - the CWS first, and then the cylinder. If the CWS doesn't empty, then there must be a blockage in that feed pipe betwixt it and the cylinder (via that gate valve).

So, if the hot cylinder empties and the CWS doesn't, you could try shutting orf that gate valve and undoing it on the cylinder side (have pans and buckets ready...) If you can detach this and after any residual water in the cylinder has been drained off, try opening the gate valve into a container and see if it drains now.

If it does, that shows the feed from the CWS to the cylinder is ok (gawd knows what else is the cause then!)

If it doesn't drain - if only a trickle comes out the gate valve and the CWS is still full of water, then you've found the cause - either a blocked pipe or a blocked gate valve.

I guess in this case you'd need to drain off the CWS (chuck a hose in there, suck and get a syphon going) before then undoing the other end of the gate valve (there WILL still be water in the pipe).

You should then know if it's crud in the valve of a blockage in the pipe - which can be shifted by attaching a 'mains' hose and blowing it back up in to the tank.

If a blockage in this supply pipe/valve ain't the cause, then there's the possibility that there's crud/insulation actually inside the hot cylinder and is blocking the outlet - I've read on here (many moons ago) of this happening...