Thanks everybody for taking the time to respond. I'm hearing from you all that buying a call blocker (or, by inference, perhaps a new phone with call blocking) is preferable to using a paid for call blocking service both in terms of effectiveness and cost over the long term.
The issue I have at the moment seems, for the moment at least, to have gone away, but should it return I'll definitely investigate the call blocker.
Just one more question however. Assuming I have the blocker set to stop all overseas calls, which is where so much of these nuisance calls originate, does that mean that the caller will get a "number unobtainable" response, and my phone will not ring? Does it record a log of what calls were blocked?