If I get a bicycle rack which is tow bar mounted (Will only carry one bike most of time) do I also need to get a light board??

If I do what else will this involve? I currently do not own a tow bar or rack.

If the items being carried cover the lights and/or number plate then you should get a suitable board.

Surprising number of illegal (lights/plate totally obscured by bikes) on the roads. Presumably plod is too comfy in his nice warm camera vans to do anything about them. 2012 SDV6 - it's missing a couple of cylinders
2008 TDV8 - it was a labour of love and is much missed

Depends on the sort of rack you get I guess, a lot have the lights and numberplate built in. (http://www.halfords.com/cycling/bike-racks/towbar-mounted-bike-racks)

If you've not got the electrics on your car, then you'll need the kit or them fitting, but it's something to consider as 1) saves the agro of having to attach a separate board to the back avoiding possible damage to your bike 2) allows you to pull other items, trailers etc and 3) just looks a lot nicer on the back of the RRS  ON THE WAY: BMW F21 M140i
MERCEDES GL350 - Metallic Navy Blue w/Grey interior

GOING: BMW F31 335D MSport+ xDrive Touring
GONE: Land Rover Discovery 4 HSE - Stornaway Grey w/Black Interior (wish we'd kept it )
GONE: BMW F10 520D MSport - Carbon Black
GONE:MY12 RRS SDV6 HSE Sumatra/Ivory/Grand Black Privacy glass

I had one of the Thule ones which could be tilted downward to allow access to the boot even with bikes attached.