OK so I have an update with good news!
I did a factory reset on the firmware i was using which was the latest of johns LTS (374.43_27E5j9527) and then immediately ran a speedtest without changing anything. No joy, same 230mbps limit.
So decided to try the latest merlin fw (380.68_2) and lo and behold I've now got full speed!
Interestingly on the LAN - SWITCH CONTROL page it only shows CTF is enabled:
On the new version there is a nice cpu graph too, so you can see the cpu load whilst doing a speed test. Much better:
Guess I'm going to have to return the new router I ordered last night.... oops !