OK - here's where I am now.
I tried the old setup direct to the test socket with a BB splitter. Made no difference to the speed with the original cable and still got a 12mbps drop when I swapped to a 5 metre length. The NTE5c is therefore ticked off as 'OK' I think.
EE sent a new Superhub to replace the BrightBox2 I had. I've now set this up and had to spedn some considerable time setting everything back up as the 'backup' from the old device obviously didn't work in the new, and configuring that was very much a learning curve I can tell you... Took me over an hour to get things reconnected here. But that's not the issue.
The SuperHub came with a longer RJ11 cable, it's about 3 metres, and I've used that whilst setting everything up. The speed I am getting is similar to what I had before, and without using a 5m length as yet. I'm wary of upsetting 'external forces' by reconnecting the router too often.
I was about to try the longer cable when the EE service went off on it's own... I'll retry later.
EE Fibre Plus