What is a good surge protector for my new iMac

If you live in a community where the power lines are buried underground I double if you need a surge protector.

I use a TrippLite multi outlet/switch protector

mainly for turning on and off external HDs, the computer and self powered docks all at one time. Our power lines are buried so the only unscheduled outage in the last few years was when a car hit a distribution transformer a block or so away.

I am using a simple upright extension lead type with multiple sockets.

You need to ensure that whichever one you chose it is compliant with your local regulations

I'm in the UK and I get relatively few power cuts so I've no way of checking that it is protecting my equipment.

I have seen surge protectors which include a monetary warranty if any damage occurs due to a surge.