I have just replaced my old Mk1 Octavia estate with a 2 year old one, problem is it doesnt have a towbar on it. I have sourced a secondhand towbar from a car the same age via ebay complete with all the wiring and relays etc, question is will I invalidate the warranty on the car if I fit it myself? I used to fit towbars for a living so dont expect to have to many problems (got a fitting guide off the VRS forum and wiring guide from PCT's website for the bypass relay).

Only if your work can be deemed to have caused any future warranty claim - so unlikely with your background