It was with both.
I just confirmed that I'd performed the speed tests/BT wholesale etc. through an ethernet and wireless connection for best results.
After the engineer has now just reset the router and performed his tests, I am receiving 67Mbps download through ethernet/power line, but WiFi still remains between 30-40Mbps.
Because of this, the engineer said it's probably a faulty router, resulting in slow Wifi throughput.
The two WiFi bands are already split. I did this before I even had the issue.
BT are sending me a brand new router, should be here by Friday.
I'll give that a try and see what happens.
Hopefully it's just a fault with the router, the engineer said he wouldn't be surprised if it was!