I have a Ideal Istar ?

On an Isar, alternating F & L indicates Flame Lockout. (Either failing to light or failing to ensure flame is established known as rectification). Firstly check that gas is available, sounds simple but you'd be amazed the times that it isn't (Emergency Control Valve at the meter turned off etc).
It could also be a blocked condense pipe (the plastic pipe that comes out of the boiler) follow the pipe to its termination and make sure the end is clear of blockage or debris, both are easy for you to check without too much trouble.

If gas is available at the boiler and the end of the condense is clear, it becomes a matter internal to the boiler, things wrong could include:
Gas valve, spark electrode/rectification, internal condense blockage, pcb etc. It would need investigation by a qualified engineer rather than attempting repair yourself.

Carefree
July 2010