It sounds like you have a faulty or incorrectly adjusted component that is not letting the boiler complete its ignition sequence.
Save yourself a lot of grief and get an engineer to look at it. If you haven't had the appliance serviced in the last 12 months, this may be a good time to do that as well. Many ignition faults are down to poor servicing.
This repair should be carried out by a Gas Safe Registered engineer who has HEATING registered on the rear of his ID card AND has experience with sealed systems. If his ID card is in the name of a company, he is only allowed to carry out this type of work when working in the pay of that company. NOT doing a cash job.
TheHeatingMan
April 2010