Hi all, thanks for the messages - indeed, she's the 5.0 ally engine; pre-curser to that in my 500E that served well for 12 years and 150k miles....!
We were away for Christmas - here's the couple of photos of the replacement belt and obvious problem with the tensioner...!
Thanks Grober for the info on the best option - will check it out today.
Best to all for '23, keep using the petrol - it won't be there forever!
Paul