There’s no markings on the hub, but it’s on a Kona Big Unit,so I’m assuming it’s the original Formula single speed rear hub.
It looks like this.

What tool do I need to fit that lock ring so I can change the sprocket ?

A lockring tool will take that off:

http://www.velosolo.co.uk/bbblock.html

you will obviously need a chain whip too.

OK,thanks.
I’ve got a chain whip and I’ve got a C spanner, or lock ring tool at work.
It’s just that the lock ring looks so thin I thought there would be a proper tool with more engagement points to avoid distorting it.
Something like this BB tool.

C spanners tend to crush lock rings oval. I guess it depends on how tight it is.