The seven working days is unreasonable. Grossly unreasonable because you can actually just have been on a fortnight’s holiday. Indeed, the only send one letter is unreasonable because you may not have got it for whatever reason.
Your only remedy therefore if they will not pay up is to take the compound to court for the cost of the vehicle which they have a reasonably disposed of.
I would exhaust the complaints procedure first and then tell them, if they don’t agree to pay up what it was worth by a deadline, take them to the Small Claims Court.
The new Moneyclaim beta system is easy to use: https://www.gov.uk/make-money-claim
the new system is much better than the old government Gateway system.
You can prepare the proceedings now, at your leisure, save them, and then just pay the fee and press the button when the deadline in your letter before action, expires.
It's then just a case of following the court process and responding to any emails that you receive.
Please note that the value is the value as it was in the compound as stolen and recovered, not the pre-theft value.
Thank you for letting me assist you with your legal question. I am glad that I was able to help.
I am not certain whether that answers the question for you or not, but I am happy to answer any specific points arising from this.
It will be my pleasure to help you again either further with this or any future questions you have