"a fee of any sort to join say the MU would seem pointless" -
Alan, how much is your box worth? How much would it cost to insure it to take outside of your house? The MU-based policy covers it even in the car (as long as it's out of sight) - and sometimes you do need to do that even if you're not playing for money - paying for fuel for example.
I haven't got the figures to hand, but it is cheaper to join the MU and insure through them than to buy a policy from someone else. That's economies of scale - and specialist knowledge - for you. An FPO is never going to achieve those numbers because they include all the instruments and equipment from classical, jazz, brass bands, rock bands... you name it.
So no-one who ever takes instruments out of the house should think it's not financially worthwhile to to join the MU. And if you have particularly nice instruments you never take out, but need to be named in your household policy, then again the MU is worth joining IIRC.
And you get all the other benefits of the union into the bargain.