Do I need insurance?

Yes and no =)

Liability insurance will cover a bocker against claims from people or companies that have been physically damaged by you in anyway while bocking.
You might ok, I can't see how I'll ever injure someone else or property so I don't need it. That's entirely up to you.

There are somethings to consider though. Stilts are made of aluminium. You are using them in a fashion that requires great balance and the result is jumping quite high in the air.
They've also been known to fail. In a rather unique incident, Welshbocker Bobman's spring went flying 30 feet away from him. If it had hit a person it would have injured them quite seriously.
Gym sessions are another example. Someone trying to flip could completely miss the crash mat and their stilts crash into the floor. Sports hall floors costs thousands of pounds, and you would be liable to pay it without adequate insurance cover.

Individual liability insurance can be purchased from various sports insurance companies for around £40-50 per year.
Details can be found in the RESOURCES section.