I am concerned about them wriggling out of a claim on what is effectively a bolt-on to the main house policy.
My strategy, if there's anything unusual to insure, is to use a broker
It costs a bit more through a broker, but if a claim is made they don't get messed about anywhere near as much as a clueless individual does, plus the claim paperwork should all be done by them, once a signed statement form has been submitted by yourself
Don't forget to tell them anything, the broker may not be an expert in insuring private hobby workshops, so write it all down, the structure, locks, contents etc, and bring photos if possible, to show the broker what's being insured
All a bit of a fiddle really
People in the USA have to do this with their private health insurance each year, each contract is unique and it can take days of effort to check what's covered and more importantly, what is not
Most people just take the easy instant bolt-on option and offer a silent prayer up to the god of insurance payouts