Thanks Kev, I have been told by the company that did the remap that it is now fixed, a "lose wire" apparently. I will be asking questions regarding the lose wire, location etc and will put on here for future members to review as may help them.
An update from me too. I ordered the sensor (5Q0 906 207 B) from Cardiff Audi on Friday and it arrived this morning. It took a few minutes to remove the old sensor (I think I was being too careful not to break anything off) - the hardest part for me was to unhook the latch on the sides, but installing the new one was super easy.
Both brake servo and stop/start warnings disappeared immediately after the sensor was replaced.
All thanks to everyone on this thread and especially Ambrose whose posts I found extremely informative!
Obviously I'm not impressed that these parts are failing in the first place (or that they could lead to bigger safety problems), but I am hugely impressed by everyone on this forum coming together to help others. I would have been without a car and few thousand pounds poorer otherwise!
Do you have roof bars for the Saloon or Avant?
On my avant the roof bar feet clamp onto the roof rails.
On the saloon the applicable feet will clamp under the roof ionto the door frame.
I have been checking ebay but no luck unfortunately.
I have however found them available on https://www.onlinecarparts.co.uk/ at a really good price.
Thanks
Update- local dealership can't see me for 4 weeks and doesn't have the part in stock but Audi Assist are coming out tomorrow morning to have a look at it.