Am I the only one here whose first thought on reading "flooded car" was carburettors? Or am I just too old?
Anyway, whatever their offer might be, you should start your homework now. The idea is to provide solid evidence to justify the full amount to which you think you're entitled.
I had a similar issue a few years ago when my motorbike was written off. (I was almost written off, too, but thank God for the ambulance and NHS). The first offer was an insult, so I created a dossier about an inch thick and sent it recorded delivery to the loss adjuster. The dossier contained my original purchase receipt, a receipt for every penny spent on it since, plus dozens of adverts from Auto Trader of similar bikes. On a separate piece of paper with each advert, I explained why my bike was worth more or less than the advertised bike.
They stopped arguing very quickly!
Good luck
