Vokera Boiler Issues

Hi, I have a Vokera Excel 80SP Combi Boiler which has recently been playing up. I'm not sure if the issues below are related but I'll include them for completeness. Any suggestions would be appreciated.
1. There is water leaking from the auto air vent. The leak only seems to occur when the central heating is off. If I turn the thermostat up so it's calling for heat all the time it doesn't seem to leak? I've bled all radiators, and tried closing the cap but it still appears to leak regardless (perhaps the cap is not sealing, but I believe it shouldn't leak water anyway?). What would cause a leak, a broken valve I would guess, but any other causes? I have a radiator off at the moment for decorating but the inlet and outlet of the pipes are sealed. I have thermostatic valves on most radiators, but there is at least one with a "standard" valve and it is open (I know there has to be some route to circulate water).
2. The pilot light is periodically extinguishing. The thermocouple was replaced a few months ago during a service, so I don't believe this to be the issue (though I guess it could be). Again if the thermostat is turned up to call for heat constantly it doesn't extinguish (have tried it for 2 days), but with the thermostat on normal it goes out, usually within 24 hours. I don't think it's the recent wind either since it has happened on calm days. Is there any way it's related to the cause of the air vent leak issue?
3. Probably of no relevance, but I recently replace a mechanical heating timer with a Drayton RF thermostat, so it's probably "on and off" more regularly than normal. Like I say probably not relevant, but mentioning it for the full picture!
I would have liked to try turning the thermostat right down to see if any of the symptom occur when it never calls for heat, but it's just too cold :D
Thanks in advance,
Simon
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Firstly does the boiler keep losing pressure? If so when the boiler is cold and the pressure is set to 1 bar(14psi) when it has reached temp how high does the pressure go?, If it rises above three bar it is more than likely the expanssion vessel. the leak on the auto air vent is common and a cheap fix is possible. let me know the outcome pete Simon wrote:

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It is losing pressure over time, but I'm assuming that's because of the air vent leak, so the system is therefore not fully sealed.
I'm not aware of the pressure going high (haven't checked exactly how high, it's in the attic so hard to get a "quick glance") but I don't think I've ever seen it get higher than 2. There's also no evidence of "blow off" on the overflow outside the house.
Interestingly over the weekend the boiler has been behaving, but the pressure has been at only 0.5 bar. I don't know whether it was just coincidence but recharging to a cold 1 bar and the pilot extinguished twice since yesterday teatime.
In terms of the auto air vent being a cheap fix, would that be to buy "Vokera 4254 Auto Air Vent" as listed here?
http://www.keeptheheaton.com/acatalog/Vokera_Misc.html
Thanks for your help
Simon
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gasman pete wrote:

> Firstly does the boiler keep losing pressure? If so when the boiler is
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