Hello again forum,
To be honest, I think it's actually gone away... then again, I have been revelling in the fact that last week I remembered where I'd written down the radio code, and have been basking in the glories of the BX's tinny speakers.
On the other hand, since fixing the cooling system, the noise doesn't seem to be as prevalent. Could that blockage have been causing the water pump (as it's in the area where the noise is coming from) to be put under strain? I think it could. Remove blockage... things are flowing nicely again.