Does the RRS come ready to plug a trailer into?

its a hse 2007

If not is it a big job to get it wired up? I only need to use it a couple of times towing my kit car for repairs

You might find the car has been spec'd with it.
Take a look behind the towing eye cover and see if the plugs are there.
If not, you can get the wiring for around £100 I believe and it is fairly simple to put in (according to other people, far too technical for me ).
I recall there is also something about activating it via the system, but I'm sure someone more knowledgeable will be along soon.  ON THE WAY: BMW F21 M140i
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Fitted a set today, took little over half an hour. If you go for the genuine electrics, they should be enabled by a diagnostic computer capable of modifying the cars CCF. They will work even if you don't enable them, but it will stop the PDC from coming on when you have a trailer connected, and similar other things

If you opt for the aftermarket electrics, they take a bit longer to fit, but don't need enabled. Your PDC will still try and work though