I need someone with a very clever mind to solve this one for me. Our CH was off all summer.  When we came to switch in on for the winter the boiler wouldn't fire up and it was because the primary system had de-pressurised.  
We have a glow worm boiler in the basement floor of our house.  On the top floor we have a Megaflow tank which I understand uses the primary CH heating loop to heat the hot water of the secondary system through a coil in the tank.  There is also a small red expnasion vessel.
For some reason we keep losing pressure in the expansion vessel rapidly and have seen intermittent (sometimes large) volumes of cold water coming down the external overflow pipe at the side of the house.
With the heating on last night the pressure reading on the expansion vessel was falling from 1.4 to 0.8 bar every 15 minutes.  It fell to zero overnight with the system OFF. There can be nothing coming down the tundish when that happens.
So far we have paid out over £750 and have acquired:
new pump
new pressure guage and pressure release valve on the expansion vessel
three new temp/pressure release valves on tank etc
14 new radiator valves (some were a little leaky)
new auto bleed valve
a system full of automicro sealant.

There are no evident damp patches/puddles but we are still losing pressure rapidly.  What the hell do I do now????? Skint and losing faith in plumbers.  Also have two very cold children!!!!

The vessel itself hasn't been checked as such (although the air side was "repressurised" with a bicyle pump).  The maximum pressure we can obtain at all is 1.4bar.  It never rises at all when the system is on. It loses pressure whether the system is on or off.

Thanks.  I guess that is pointing me towards a pretty significant leak which I suppose the sealant stuff has little hope of solving.  This is a fairly new house (~6 years old) and all the pipes are hidden. There are no clues at the moment. I'd have no idea where to start looking.  We can't afford a massive wild goose chase either.

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I really must learn to read posts more carefully.  Scratch my last reply.

If that's still occuring and you are sure that's the water is coming from the PRV then either the pressure must be increasing and causing the PRV to open or the PRV is faulty.
 
Could still be an expansion vessel problem causing the pressure to rise as thescruff says.

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Is that because the water main pressure is 1.4 bar or do you get water coming out of the PRV if you try to use the filler to raise the pressure beyond 1.4 bar?

Nothing to do with the problem, but why the heck wasn't the megaflow installed in the basement with the boiler.  

Just checked...its a 15 litre vessel which has a max working pressure of 4 bar so that seems Ok.

The pressure/temp relief valve on the hot water tank was changed but I think I have been taken for a ride there as (now I am beginning to understand this) it would appear it has nothing to do with the problem at all!

The boiler doesn't seem to have a TPR.  It has cold water in, hot water in, gas in only.  There is no evidence of any relief or overflow from the boiler itself (expcet a gas flu to the outside).  The boile does sitch off when the pressure reading ont he expansion vessel drops (which I assume it suposed to do for saftey)

Incidentally, the origianl plumbers (whose business card sticker was on my boiler) insisted I pay before they left the property and has since given me a false address for their business.  Ironically I am quite an intelligent, educated person. Feel like a total idiot right now!

It's a glow worm MICRON.
I'm told that we have a cold water regulator between the mains and the boiler itself to take the pressure down accordingly.  The filling loop to the expansion vessel however is however directly mains fed so, in theory should be able to reach more than 1.4bar.

We had a british gas guy here today who has conlcuded

1)the last thing to change is the expansion vessel on the basis that:
a)all the leaky radiators going at the same time idicate overpressure
b)the intermittent leaks ot the outside would also indicate over pressure, at least at times
c)the fact that the pressure in the expansion vessel is impossible to drive beyond 1.4 bar may suggest that it is is falsely reporting the pressure and that we believing it to be lower than it is
d)the "dodgy" first plumbers changed the guage/valve to the vessel but may not have done it correctly (especialy since they changed two parts that have no bearing on the problem and also changed the pump which also looks like it was unnecessary)
He has ordered the parts and is coming back on Monday

If that fails

2)we are searching for a hidden and substantial leak (odd cos we have NO evidence at all of damp pataches, soggy floor etc) Terrified of the cost implications there as we have already been drained financially on thsi problem

I'm a bit baffled.  I still don't understand how a faulty vessel would cause so much air to get it?

In the meantime I got an address for the dodgy plumber from Companies House and am about to send a registered letter asking for a refund.......

Thanks for everyone's support

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Ok I'll repeat.

The vessel is on the top floor and the gauge is reading 1.4bar.

lets assume it 10m from the ground floor radiators to the vessel, that make the pressure in the radiators on that floor 2.4bar and even more in the boiler. when the water heats up that will be over 3bar.

Going on the boiler model I assume it's quite an old system.

What the BG man says about the pressure is correct to a point, but it will be interesting to know what parts he's ordering, before he fits them.

Several times the BG man has mentioned over pressure.  

Over pressure is all related to the vessel that he's dis-counting   Yet it's the simplest thing in the building to check.

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On the red vessel is a car tyre valve, maybe under a black plate which unscrews, if you depress it quickly, you either get a jet of water or  air, if water it is duff and needs a new vessel. it is that simple.

If you have several radiators with leaks, the pressure on the top floor will drop rapidly.

Forgot about the air problem before, assuming the vessel is duff and it certainly sounds like it, and as above, takes about 10 secs to check, then the numpty's pumping air in is going straight into the system.