Worcester 240 RSF Air Lock?

Hi all,
I am in the process of adding some new rads and pipework to the upstairs of my house. In order to replace the upstairs pipework I drained down my Worcester 240 today, leaving the downstairs in place for now and removing the upstairs pipes.
Over the last few months I have drained and refilled several times to do work with no problems. But today after refilling, the boiler has decided that it doesn't want to fire-up again.
The pressure is refilled to 1.5 bar and holding steady.
I have checked the pump by loosening the screw with it running and got a fair bit of air out, but it didn't light.
I have read up about the AAV and took off the inner cover to see if the cap was screwed down. But there was no cap fitted at all.
I wondered if perhaps the AAV was stuck and not letting the air vent out. I certainly didn't hear any air coming out of it when filling the system up.
Should I try to shove something down the hole in case it's stuck. I have a feeling it will probably jam open if I do.
I'm fairly sure the fan is on as well as the pump, but not certain.
There is no pilot light or sparking noise for either the heating or the hot water.
The only problems before this was a small leak from the Diverter Valve which we have lived with for several years without it getting worse. Just a few drips several minutes after demand for HW.
Any suggestions please?

It sounds as if someone tried to use the boiler when you had drained it.

I suspect the o/h stat has tripped as a result!

Tony

I thought it might have tripped because of the possible air lock, so I checked it. It didn't look as though it needed resetting.
I think I was looking at the right thing. Top right, near heat exchanger, black plastic with red dot in middle and something like three wires attached? Does this sound about right?
I assume that the bit that looks like a red dot pops up when it has tripped?
I'm wondering if it's the fan that's gone. It made some pretty bad sounds when it went through the motions the first time. Perhaps I should turn the gas off, remove the inner cover again and try running it that way so I can feel better if it's running. Last winter it used to make a dreadful noise when it was very cold outside, until it had been running for a while.
Seems an unlikely coincidence though.
I'm sure that the boiler must be at least 15 years old and have no doubt that the whole thing is full of sludge and rust anyway by now. But I simply can't afford to replace it without trying to fix it a bit first myself.
Any other ideas please?

Ah Ha! So that's the reset button then!
Obvious once you know that it's disguised as a plastic threaded screw I guess! Not the thermostat that I had been proding then!
Problem solved. All working fine again.
Many thanks.