Retrofitting ISOFIX mounts - pch
I have a 98 Audi A6 Avant that according to Britax is compatible for ISOFIX mounts. However my car doesn't have them fitted and the local Audi dealer says that for safety reasons they should only be fitted at the factory.

I note in these pages that ISOFIX may have its problems, and I'm wondering if it's worth pursuing or should I just stick with seat belt restrained seats. Any thoughts?
Retrofitting ISOFIX mounts - Dave_TD
I've been using seatbelt fitted seats for the last 4 1/2 years now, my Skoda Octavia estate has the ISOFIX mountings but they don't look man enough for the job to me. As long as you take the time to ensure your seats are properly fitted to the car (roadside checks have shown that most aren't) then you shouldn't have a problem.
They all have to meet the same safety standards, regardless of which mounting method they use.

I'm hoping for a negative response to my next question, but does anybody here have experience of an accident in a car with a child-seat fitted? How did the seat perform? How was it mounted?

My two have nearly outgrown child seats now, but they have been taught to make sure they get strapped in properly, and they certainly let me know about it if I look like I'm as much as going to start the engine before they're happy!