I have been asked by Free Reed to consider putting a few lines together on this very subject for their projected release of an extended reissue of The Transports. I knew Peter well and was very fond of him. Let's say that playing the Anglo was not his strongest suit .
Thus spoke Roger Digby on concertina net.
I have to disagree strongly with this statement,Peter Bellamy in my opinion was a very good accompanist on the Anglo Concertina,he had been a member of Young Tradition harmony group ,and he brought his knowledge of harmony to good use on the anglo.
As someone who has gone into song accompaniment in some depth [on the English concertina]I would advise budding concertina accompanists to give Peter Bellamy a listen,I would also advise studying chords,harmony ,chord substitution etc,much as a guitarist might do.
as regards song accompaniment, anglo playing was his strong suit,he accompanied himself very well,I notice that Brian Peters[an excellent anglo player had the following to say
Being a new member I missed this thread first time around, and am not quite sure how come it's suddenly been revived after going way off topic. However, for what it's worth, here's what I think:

Bellamy was technically a relatively limited player. As far as I know he never played instrumentals, only song accompaniments. His approach to anglo was very much "along the rows", ignoring the top row and the reversed duplicates so that when he picked out a melody, it was in a very jerky, in-and-out style. He also didn't go in much for sustained chords behind the vocal line, so his accompaniments tend to sound pretty busy.
Having said that, while they could a little clunky on some songs, on others they were devastatingly effective. Whether through musical theory or (more likely) trial and error, Bellamy could conjure up the most haunting, often dissonant chords which added whole new layers to songs like his version of "The Housecarpenter" (originally on Both Sides Then) or "My Boy Jack" (on one of the Kipling albums). Chris Timson was right about his use of drones, but the clever bit was in holding part-chords down at the same time as the drone, resulting in interesting clashes. My memory is that it was Peter himself, not just his imitators, who had clips fitted to the instrument to hold down the drones, at least one of which was located on the left hand top row, not the usual thumb button.

I could go on at greater length but won't for now. Stirrings magazine (www.stirrings.co.uk) published a Bellamy special issue a few months ago, which is worth reading if you want to know more about an enormous but controversial and fragile talent.
http://www.dickmiles.com