So a bump update on my experience.
I collected car just prior to xmas having had a significant manufacturer contribution (over 10%)
I competed all the "paperwork" online and via email etc, PCP was electronic online, a few bits of paper in the dealership on collection day, but this was their own internal stuff like signing I had been offered allow insurance, etc etc
After having the car almost 2 weeks and deciding I almost certainly won't be just handing it back in 3 years I found a personal loan for half the APR. Took this out for a longer term and got the funds into bank.
I then sent a message to Audi Finance (really VW finance) noting my intention to withdraw. I got slightly jittery they may not accept that as "written" so called them instead. Updated some details on their system over the phone and after reading a very short statement they asked to confirm I still wanted to withdraw, i confirmed and they said thats it, i would get an email (my choice as opposed to letter) with the details and I had 30 days to pay.
Emil didn't arrive so 24 hours later I rang again and it seems the first operator missed the email address so it couldn't email me, they took details and said would email me details, email arrived 5 mins later.
I realised this morning they hadn't said if I needed to quote something specific on the payment, but before I had the time to call them, they called me, they had conveniently picked up my initial message and were calling to confirm I wanted to withdraw, so I said yes but I already spoke to 2 operators, but could they confirm the reference to quote, they did.
So considering I spoke to 3 operators I must have spent max 5 minutes on the phone. At no point did I get any hard sell, and horror stories or anything to persuade me to not withdraw. Completely seamless and easy going.
So something I was a little nervous about ended up completely painless.
Saved half the interest and pay over 5 years rather than 3. Only took a loan as I am getting better returns on investments than the rate I am paying