Personally I ALWAYS believed it was the insured persons responsibility to check the details - especially when the policy documentation arrives on the doormat, even if the insurer seemingly insured the vehicle with correct details being provided.
When the internet sites first came up a few years ago they often had an incorrect or incomplete list of models or excluded imports and so it was always best to double check - and especially if a quote looked TOO cheap. This is why I personally like specialist insurers where you can speak to people over the phone, discuss modifications and have your calls recorded for additional security - that is if you have nothing to hide. Under declaring modifications/car model details either accidentally or deliberately is gonna cause problems in the long runm, especially in the case of an accident.
The rising cost of insurance though is a concern which p****s most people off including me ...
Martin
