Thanks, SD.
I like learning and this evening I learned some more!
I already knew about b and g coexisting dramatically reducing throughput (my home network is set to g only for this very reason) but I hadn't stopped to think that actually to have them coexist means two signals.
Whilst on holiday I found on the web the User manual for the access point I was using and it advised that b range is something like 2 or 3 times that of g. It therefore fits perfectly that I was initially connecting to a b signal given the distance and obstacles involved, hence the fall back rates you quote fitting with what I saw too.
Cheers!
XV