Insuring a car to be kept at a different address

I need to insure a car which won't be kept at my home address, but at the address of an ageing relative who can't drive it, but needs me to drive it / them on occasion when I visit. Anyone got any ideas on how to go about this? Would it require a specialist insurer or broker? The usual comparison sites don't seem sophisticated enough to handle this sort of requirement, and I worry it would be too easy to take out insurance which was invalid as they'd not understood it properly.