After looking around I think I'm going to go for a Zumo.
The features I need are :
Waterproof
Bluetooth audio or headphone socket
Basemap covering UK
Ability to upload SMC topo maps (limits me to a garmin product)
Spoken directions.
The eTrex and similar don't do spoken directions.
Most Nuvi's aren't water proof and those that are do not have bluetooth or headphone socket.
Streetpilots aren't waterproof (and might be locked to road use?)
Only the Zumo's cover all my needs. It's like they planned it that way.
Unfortunately they don't come with 4.3" screens and cost about x1.5 the price of a similarly featured non waterproof model.
Ah well, it is only money.