Thanks, I've started going through the information at http://www.regular-expressions.info/, but the learning curve is pretty steep. I tried a few changes, but they would fail on either type. I actually am trying to decifer your expression in plain english but can't get how the question marks work in this case.
Are there any other references I should follow? I'm starting to realise that if I get a handle on regex I'll be able to do much more clever things with MP3tag. I also use PSPad (initially because Slim Devices squeezecentre uses unix format files, but since then because it is a great text editor), and I can envisage using Regex with that from time to time.
Thanks again for your help.