It's better for the band as a whole to have cover, not just the caller or one member.
A band can join the EFDSS as an affiliate member and get the insurance cover - that what Fendragon does. The cover included whoever's calling at any particular gig, and you don't have to specify exactly who's in the band, though obviously one person has to be the designated contact when applying for membership. The total cost is the same as purchasing insurance direct from MIS (same insurers whichever way you do it) the limit of liability was lower for a time but it's been raised to match the limit for direct-purchased PLI.

Membership of the Musicians Union is also a good idea. We are, and their legal department have helped us once. Even without joining, you can also download a sample contract from their website - we did that and use it as a basis for our band contracts, and we always use written contracts - it saves a lot of arguing, and it help us to focus on getting essential details (times, fees) established before taking on a booking. Note that MU membership for insurance doesn't work too well if not all members of the band are MU members - if push comes to shove, the MU's insurers will try to make the band's non-MU members liable in the event of a claim. And unlike the EFDSS, a band can't join the MU - it's individuals only.

For good measure, we also have a health and safety policy which mentions PAT testing but also makes clear our own requirements of the venue. We send a printed copy with contract.

Anahata