I have fired up the laptop for this one..... normally i post on my phone but i need to be clear here...
Firstly I am not just a know it all...my day job is Type Approval ("e" or "E" marking of vehicle components, and Tow bars is one of my specialist subjects..)
Heres the basic deal..
If the vehicle is not homologated to tow, then legally you can not fit a towbar, even if you are not using to tow a trailer and just to attach a cycle carrier, it does not matter, the potential is still there to hook a trailer up.
Now, if your vin plate on the vehicle list a number of weights, inc a gross train weight which is higher than the vehicle Gross Vehicle Weight then you can tow a trailer.
When it comes to old Minis, VW Campers etc these can have a towbar fitted because 1) the manufacture said they can tow... and 2, they don't have Whole Vehicle Type Approval.. meaning no coupling approval. (If the vehicle has Type Approval then in order to be able to fit a tow bar that vehicle must have a Coupling Approval.. which basically means that the manufacture has said a bar can be fitted, and when it is fitted it complies with the rules on tow bars)
If you fit a tow bar to a M1 passenger vehicle in the UK (I.e. a car) it has to comply with "Construction and Use" This is national law! C and U states the bar must be Type Approved... so by fitting a bar to a vehicle which is NOT homologated you will be in breach of C and U.
(Trailer now have to be Type Approved too, so people building their own home made trailers - not ones that are already in use - should be either getting Whole Vehicle Type Approval, a National Type Approval, or for One Offs, they should be taken to VOSA for a IVA (Individual Type Approval)
The normal Leon bar will fit... however your insurance will be null and void if anything happens..... as will VOSA..
As for your V5 not listing weights before... depending how old the car was, and what info was placed on the DVLA system by the dealer (dealers have access to register and enter some info about the vehicle, the rest is generated from the vehicle type approval info held on file I believe) it is very likely that info was missing...this is very true if it is a older car, as each time the V5 is updated they add more information, however the vehicles that already have V5s dont get extra info added, they just get blanks..
however from the tone of your post it appears you have already made your mind up, a bar is going on.... but maybe others will heed the advice?