Found a way around this - helped by the fact that the amp was at rib-rattling volume by half way around the dial. I bought some Rothwell Attenuators, which don't seem to affect the audio signal at all but reduce amplitude by 10dB (15 & 20 also available) so I now use more of the dial, but the part I am using is spread over a larger amount of the dial - and the smallest increment is now where I want it to be.

Thanks for trying again though!