I'll post this here in case it helps someone. If you need to get power to the Openreach Nokia ONT but you don't want to run a mains extension - this is the appropriate 12V DC extension cable you need (which I discovered from a bit of trial and error):https://www.ebay.co.uk/itm/383423097541?var=651852504222
In my case I only needed 1.5m extra length so I don't think that would be a concern for me.