At this point you may be in a sort of Catch-22. The fan only runs while the boiler heater element is active. So if the fan is the reason that your boiler failed you won't be able to tell for sure if the fan works or not until the boiler problem is resolved.
During the period of time when you first turn on the S1 up until the error lights comes on, does your boiler lamp ever blink? If so this is when the fan should be running.
BTW: Even in 20A mode, when first started up, the group heats up first before the boiler fires up for the first time. Only after this initial warm up can the boilers turn on at the same time. I assume this was a safety thing since this is the only time when both boilers would otherwise be on for an extended period. When the boiler would first turn on to heat for the first time, is this when you get the error lamps?
If you're a tech geek
and have a ohmmeter you can always check the boiler heat element. You should get some low ohms result but not zero or open. Also if you have a 12v battery (a 9V might even work) you can hook it directly to the fan to see if the fan spins. Both of these with the unit unplugged of course!