Chas,
When I was starting with Anglo and also when I later started with melodeon I found the major problem (for me, anyway) when trying to sing with them, was that the instrument required a "breathing pattern" that was at odds with the one my voice required. Plugging away at it has allowed me to get a few accompanied songs off so, like much else in life, I suspect it just requires practising it.

Cheers, Rowan