Optia ff50 boiler fan cuts out anyone know what to do?

There is a problem with the fan situated at the top of my Optia FF50 boiler. When the boiler is first switched on the fan comes on and it makes a horrible squeaky noise whilst running. The boiler fires up for a few seconds and then the fan cuts out and as a result the boiler goes off. The fan motor is hot so I am sure that it is a problem with the fan motor. However it is in a sealed unit that I cannot get into.
Does anyone know anything about these and whether it can be repaired at a low cost? Any advice would be much appreciated.

Tim