As Tinker says, you need either an audible or visible warning that the trailer indiciators are working. The light should flash in time with the indicators or the buzzer should sound with each flash. You can wire the car without, but legally one or the other is required.
A buzzer is easier to install, and the way that most installers would do it these days. I'd simply buy a trailer flash buzzer, fit that and not bother to install the lamp. Here's one on Towsure - only 3 quid, but the P&P may well come to more. You'd be better off getting one locally if you can.
If the Concorde is RXE spec, then it will have bulb failure warning on the brake and side lights. I don't think this extends to the indicators (you're expected to notice the rapid flash when a bulb is out, like any other car.) I don't think the reversing lights or rear fogs are monitored either, but don't quote me on that
Having said that, I wired both of my RXE's conventionally, with no special consideration for the bulb failure warning. The only problem I ever had was that if the trailer plug wasn't in properly, resulting in a bad earth return to the car - the voice synthesizer would report a side light fault every time you braked with the side lights turned on.