Thanks all, yes I appreciate it seems mad to take it all apart, but did this 10 years ago with a Saxo having owned that from new for exactly one week and all was well for the remaining seven years.
It doesn't look too hard to get into - removing the seats looks OK, the rear quarters might be tough, will only take out console by handbrake etc if I need to, aim is to just lift the carpet one side at a time.
Thanks for the info on the sound. I'm sure that is right, after all you only need to open a window a tiny bit to get a lot of noise in.
I took some measurements this morning, 60mph 5th gear along my reference bit of road, the A249 north bound near Detling, 70-74dB(A) and over 90dB without the filtering. Will be a few weeks before it's done but I will post back.
I suddenly wondered whether the extra gram CO2 that enables the Diesel to get in the £20 tax and CC exempt was achieved by weight reduction, specifically the sound proofing. That might explain why others are finding it OK, if they have a 111g/km diesel or a petrol?
If anyone would like to borrow my sound meter do let me know - happy to post!
Thanks.