I'm trying to lead you down the path of fault-finding, instead of diasnosis by conjecture, and I would say you (and I) don't have information to say with any certainty that the pump is not powerful enough, especially as the pump is contained within the boiler casing (as is my understanding anyway).
The fact that the upstairs rads are hot doesn't mean that the pump is running - this can happen by convection. Moreover, hot upstairs and cold downstairs is a classic sign of failed, worn, or inoperative pump.
With 11 rads, I presume that you have 6 up and 5 down, but how many downstairs rads are becoming what you would call hot?
If none, then the pump is number one suspect, but before you replace it it would be sensible to check that it's actually running. If is isn't, you'll need to check that pump has an electrical supply, because the failure could be elsewhere.
You say that it feels as though it's running - again, how are you testing for this?