Thanks for the tips guys! MTB shoes would be a good solution as Staehpj1 says, but up to now I've only had one pair of "running" shoes for everything on and off bike, not wanting to carry another pair of off-bike shoes, but this reasoning is obviously negated by the idea of carrying wellies - and yes I do mean those heavy cumbersome rubber Paddington Bear things

I'll look into booties. Any recommendations? I have heavy duty neoprene overshoes for winter riding but this isn't what I need for relatively warm multiple rainy days riding and camping. Maybe a second pair of Sealskinz socks to change into at camp would be good enough ...