Thanks Kevin. Yes it must be gold (!) - the dealer said the replacement DPF unit was £3,500, and when I queried how long it would take to fit they said 8 hours - so I then queried £3,000 for 8 hours (?!) and they said there are other things that also have to be replaced like sensors and hoses, and a computer reset etc.
The dealer rang me first thing this morning and said they could 'help' by reducing the cost to £5,200 - so £1,300 off 'to help' - which is a start, but £5 grand is still unaffordable for me. I wonder if it's because I told them I would have to try and find another solution and took the car back off them.
Off to see an independent Audi specialist today. Spoke to them on the phone yesterday and they said a clean might work, but they would review, test and then discuss it with me. I've also been told by someone else that I could consider a 'remap' - whatever that means. I'll try the specialist first and see what they say.
Watch this space...