Based on 65k:
CAP retail - £13,595
Glass's Guide Retail transacted - £11,780.
These should give you some numbers to work with.
I don't really think you would get £15k, but then again I don't know what the mileage is. Remember that the insurer will only pay the market value of your car, not what you are replacing it with.
Negotiation is what it's all about but remember be prepared to back up any valuation you put on the car with ' like for like' market examples ( Auto Trader ) or Guide figures.