I have purchased my 68 plate audi a6 estate 40 TDI last week. I have had numerous quotes for a towbar an had the expensive prices for which have been told need module an programmed to the car which is around 800. An then universal which is around 500. What is the difference? Apparently if I got the module installation I would have all the extras for abs etc available. What do you recommend??

I guess the £500 is like your standard aftermarket towbar which have been perfectly adequate for years. The.key questions you need to get answered is whether it's a fixed or detachable towbar. As well as if it's a swan neck type etc. 

I believe the genuine Audi ones are made by westfalia. 

https://www.ultimatetowbars.co.uk/

Ultimate Towbars are great towbar fitters and they are mobile too so come to you at work or home to complete the work. You need to be careful who you go to to, a lot of towbar fitters will use universal electrics in order to make it cheaper but these can void your warranty and usually isn't the safest option. 

I have used Chester Towbars 3 times on my last 3 models and they have fitted an detachable Westphalia everytime with no issues whatsoever.  I think the most recent on my A6 C8 19 plate was £700 all in.