you really need to get the boiler casing off and have a look at the pump, even then just cos it spinning doesnt mean its ok....

we have a spcial program on laptops that when a special sensor is fitted can tell you how fast the pump is going.

are there any loud noises from the boiler itself?

for example a noise like build up in pressure that dies away then comes back?
(actually the water boiling in the heat exchanger cos the pump is not pumping it away fast enough)