Basses on a C/C# or B/C box are not necessarily in G/D. On the Erica for example they are in the home keys and you have to get them retuned

Interesting! The 21-button B/C box I've been playing with (thanks Ged) has a standard layout apart from one of the basses - instead of

G | D   C | G
E | A   C | F

it's got

C#| B   C | G
E | A   C | F

I haven't yet found a use for a C# or B bass note, but I'll keep looking.

I'm interested by the contrast between English concertina ('logical') & melodeon ('intuitive'). Coming to it fresh (as a whistle-player) I find the push/pull system incredibly *un*intuitive - it seems to mean developing two separate bits of muscle memory for every note. I can't see myself learning many tunes that way. It's nice for chording, but in that respect its range is very limited - I was trying to work out how to play a D major chord for about five minutes this afternoon before I realised that it can't be done. I think an English concertina is going to be the box for me, when I can afford one!