That car’s too old to justify getting it repaired at Audi, if you must take it to a specialist.
I’ve got an A3 with the same engine…if I took it to Audi it would cost a fortune to fix.
I doubt the diagnostic sheet will tell you much…it’ll have a non-descript error code that’ll point to ‘an electrical fault’. Although your Audi engineer friend might be able to glean more info from it.
If you’re anywhere near Epsom I’d recommend TVR Fix…they’re used to big engines/sporty cars and I got mine fixed there last year.