I have the same problem with an Ideal Mexico 2. The pilot light looks ok, it has a thermocouple in the flame (the tip of which is glowing red). The motorised values up in the airing cupboard open and close as hot water and / or central heating is selected. The pump sounds as though it's working ok (and you can hear the pump and water change speed when it's speed selector switch is moved).
After trying to find the problem for a while I disconnected the mains electricity and turned the pilot light off. After a couple of hours I relit the pilot light and reconnected the mains, I set the room thermostat to maximum and also selected hotwater. All seems ok except the boiler just doesn't fire up (also had the boiler thermostat up at maximum, it's normally between 3 and 4).
Any thought's or comments would be very very welcome. Thank you
Cold Kev in Herts, October 2009