Welcome Stuart!
For the price of your old car, you can try the Autotrader. They have a free valuation service. You enter your registration and mileage and it returns a private sale price and trade in price.
You can also use Autotrader to search for similar cars around your area, 40-60 miles radius is what the franchises use as an area to give you guarantee that you buy their car at the lowest price. So you can search how much these cars go in your area.
I searched on Autotrader for models from 2005 with max 80K miles and the cheapest 2.0 is £1.4K:
http://www.a1ortos.co.uk/used-MAZDA-MAZDA%206-Oldham-Greater-Manchester-963059This is a 2006 model with 67K miles.
Check the rust in the back arches and the missing tow eye in the front bumper.
Does your car look similar to this? As a private sale you will sell it for less money but you will get more than selling to a franchise.