Update
Before fitting the new transformer, I thought I'd wire up to the mains, check for humming, the secondary voltages and play with the taps.
You'll all be glad to hear - absolutely no hum I could hear (with my ear touching the transformer) or feel on any of the taps, that's 220,230,240 an 250v.
The output ranges from 34.5v to 40.5v AC (across 18 - 18V secondary taps, there is a 0V too) this is with incoming mains at 245V AC.
I haven't connected it to a load yet. I'll be using the 240V tap.
I reckon it'll need CJ14's test rig to detect anything.
I'll be ordering toroidal transformers from Stephen at Tiger Toroids for future projects.
(May be he'll do ones with black and orange stripes