How do I change the pump on a Bahama 100?

As with most people I have become very familier with this boilers little querks.
The latest problem is the water doesn't apear to be circulating properly, after the boiler fires up the temperature of the pipe with the thermostat at the back of the boiler gradualy incresses until the the gas valve shuts down until the temperature drops then fires up again.
The radiater furthist away from the boiler remains cool sugesting to me the pump is not working as it should.

Any ideas please.