Hello Kev,
I've probably missed it, but I'm not sure what year this vehicle is.
Retail value at a dealer ?
Should be easily assessed via. On-line valuation services, and complimented by searching for similar vehicles being sold by other dealers.
Part exchanging:- often a source of confusion since folks tend to get hung up about getting another £100 or so from another dealer, but are not too concerned that the vehicle they are buying is say £200 more at that dealer.
In all of this, the only figure you need to compare with different dealers is the 'price to change', ie. How much you have to add to your car to buy the car you are interested in. Inflating retail prices and part exchange prices still comes back to that 'price to change'.
Kind regards,
Gareth.