No time to look at it then, so had a poke around today. Luckily, over the week I'd had a thought. The tail lights have two separate circuits for left and right, so you don't lose all your rear lights with a fuse / wiring fault. Now the cheapskate trailer manufacturer has saved a few tenths of a eurocent by not using two circuits, but driving both bulbs from one side using 6 core wire instead of 7 core. Neither of my previous towcars had bulb fail warning on the tail lamps so had never detected this. Added an extra wire alongside the existing cable, no more fault warnings.