I am embarrassed to say that after hearing the chain slapping, I thought it was time to adjust it. Hasn't ever been adjusted by me, so I had to look it up.
That 5 minute job took just 5 minutes which has to be a first
Just one easily accessible nut to loosen and then yank the lever anti-clockwise before doing the nut back up tight.
Coolant flush and bottle expansion bottle clean is next. How hard can it be