I just replaced the radiator cap for a new KTM part. I have been loosing approximately 1 cm of coolant out of the overflow tank every 300 miles or so. I hope this does the trick.

Do you think the old one was knackered?

Thanks for your time,

I was losing about that much coolant myself at one point. Coolant was bubbling out of the top of the water pump cover, had to replace the gasket.

In 8 years already had to change the radiator cap twice, but looked worse than yours ( more wrinkles ).

But level drop could be similar IIRC so you may be in luck ( at least for now no bubbling or gasket needed to replace in my SD )