Boot not closing shut A150 (W169)

[DuncanSE]

My wife's A150 Classic SE (05 plate, I think that makes it a W169) is not closing properly. The boot appears to shut, there is nothing obstructing it, it looks shut in it's normal shut position. But lift it again without using the handle and it opens. The catch that should hold it closed is not keeping it closed. The U shape metal hasp on the floor of the boot is not loose or moved. If I use a screwdriver on the catch when the boot is open to simulate the door closing, the catch moves but doesn't stay closed. Whatever spring is inside the catch is making it always try to stay in the open position. Of course not knowing exactly how it should operate when working normally I can't tell if it's normal or not. When the door handle above the rear number plate is pressed there is a short whirring noise. Is it simply that the catch is broken?