BT Openreach have just come and swapped out the Fibre modem.
Apparently its a known issue, they are susceptible to failing quite quickly, especially if they are not vented very well (which was not the case here).
The engineer was quite cool, he said they are trialling a new solution at the moment, but all they can do currently is swap the box out for a new one.
He said he's had some only last a few days, so mine's had a good run since March
Anyone who's got Infinity Fibre, if your service suddenly starts dropping out, just tell them you suspect your Openreach Fibre Modem has failed and needs replacing, save some diagnosis time
