I'v just chatted to my broker and she tells me that it works like this:
Because my policy's renewal date is October the sums assured as shown in my post and the values as reflected in the Autotrader at present are in line. However if I prang my car in, say September 2016 and it is a write off, I'll not get the values as contained in the policy because the vehicle will depreciate over eleven months. What I'll get would be close to the prices that would be listed on the Autotrader website in September 2016.
The argument used by the insurance industry use as to why the monthly premiums don't decrease:
Whist the value of the vehicle decreases every month, the cost of vehicle parts and labour increases.
In the event of my having a prang during this period the insurer has made provision for the increase in the cost of repair.
I checked this with a panel beater and he assured me that the rapid escalation of costs of components and material to do the repairs is mind boggling.
My monthly premiums are low because of my age, my track record of claims and because I reside in a small town; folk here don't steal cars, they simply borrow them!