I think I have already mentioned that all the leaking rads have been fixed so that can no longer be the issue. The "bicycle" valve still hisses with air.

I'm dubious that the vessel is the issue.  If it isn't, does that take us back to a significant hidden leak? If so, it is well concealed.  No idea what we do at that point.  

Thanks, balloon in situ.

There is no evidence of a PRV for the boiler at all.

Have looked and looked for any sign of a drip from any pipe but there is just nothing.

I'll pass on advice to the BG guy tomorrow and see where we end up

Cheers

What is the make of the floors, assuming the pipes are under the floor somewhere.

If you have a leak, are you covered by the house insurance.

I know you said you looked for a drip/leak, but have you run your fingers around the fittings

The floors are either solid wood or laminate or stone on the ground and basement floors, all laid directly onto concrete so I think we should have seen something ???

Here's another crazy theory, tell me if it's bonkers.....

IMORTANT OBSERVATIONS
Twice now (with the central heating and hot water off at the timer control panel but with the immersion heater on), having run a bath or a shower, we have noticed that 20 mins or so later the pump is going like the clappers and clearly struggling.  It has had to be switched off at the wall socket.

We have mainly observed loss of pressure from the primary side but there have been intermittent releases from the overflow pipe over that past few weeks

The hot water tank, pump and connecting pipes are on the top floor (4 storey) in a cupboard that is raised above floor level by about 1metre.  As such all these elements are ABOVE the level of the top floor radiators expect perhaps the uppermost portion of the vertical radiator/towel rail in the bathroom. I have heard a lot of bubbling and gurgling coming from that towel rail.

CRAZY THEORY...

could the primary loop/coil inside the MegaFlo/Heatre Sadia tank have ruptured leaving a route for the central heating water in the primary system to discharge into the hot***er?  This would at least explain why there is no evidence anywhere of water loss.

I know we would perhaps expect some sediment or dirty water coming out of the taps, but as the tank etc are ABOVE the level or the radiator, could it be possible that we are just losing the clean fesh water from the top that we are adding to the system each time we top it up?

It's just a thought but we are running out of feasible options.

Can you tell us what controls you have, there should be a 2port valve on the cylinder, which is wired through the thermal cut-out. Do you then have an additional 2 port for the heating or a 3 port diverter or mid position valve.

SORRY, YOU'VE LOST ME, i HAVE NO IDEA WHAT YOU ARE TALKING ABOUT.  FRUSTRATED AND STUPID!

Which pipe are you getting the intermittent release from, if the cylinder then it is a separate problem to the heating, but again pressure related, white vessel or internal air bubble.

THE INTERMITTENT RELEASE WAS FROM THE EXTERNAL OVERFLOW PIPE WHICH I UNDERSTAND WOULD COME EITHER FROM THE PRIMARY OR SECONDARY SYSTEMS  IT WAS NEVER CLEAR WHICH ONE AS FAR AS I KNOW

You need to proof the leak is in the pipework by isolating the boiler and pressure testing with a test bucket, actually finding the leak will be another story, and could be very disruptive, although you should be able to make a rough drawing of where the pipes run.

BG ARE PLANNING TO TEST FOR A LEAK ON FRIDAY (BY ISOLATING THE BASEMENT FLOOR FROM THE REST OF THE HOUSE IN THE FIRST INSTANCE.  I DON'T UNDERSTAND THE TEST BUCKET THING AT ALL.  THEY HAVE NEVER EVEN MENTIONED THAT AS AN OPTION

As said earlier, your house insurance should cover the leak, or most of it. [/quote]

YES I THINK IT DOES.  I JUST WANT IT FIXED BEFORE WE ALL FREEZE TO DEATH.  MANY THANKS FOR YOUR CONTINUED INPUT.

I must be one of the only people in the world to be genuinely delighted to find a large damp patch!  Finally the leak has shown itself.  It was hiding behind a book case that is fixed to the wall (on the ground floor) and has extended down into the basement floor (hidden behind a tall wooden cabinet and curtain).  The wooden floor has also lifted but it was underneath a large heavy sofa.  Eureka, I think we've found it!

Yup, Good news.

Who nailed the bookcase to the wall?