Hi guys, just wanted to thank you for all your help and advice. Tubes have been changed, and the offending noise seems to have disappeared. Hurrah.
I did run into a couple of issues whilst biasing the tubes;
The meter I used only had settings for 250m or 100m. The guide said I should be using 200m.
I couldn't get any reading at all whilst in A/B mode.
In A mode I got a reading of 9.0 for both valves (I tested it twice, once in 250m and once in 100m. Same reading both times)
So I decided not to adjust the bias at all, considering I wasn't in the right mode etc, and it did seem to be balanced.
I've just been playing for a good half hour and everything seems fine....maybe a slightly louder hum, but that could be my imagination, or the new tubes.