Just a little thing i took from your original post. They are aged relatives.
I would guess from that they would appreciate a bit of service and support from a happy dealership, maybe above and beyond which would come from the dealer getting a good deal as well.
Lets face it, the car isnt high premium car so the margins are likely to be pretty small. I think you would be better served making sure they offer them the best of service rather than just a few quid off.
My dad is ex forces and wouldnt get stiffed by anyone. Buys his cars of the same salesman every time and never ever queries the price even though he knows i could easily get him a better deal. Why is this? Because he knows if it coughs or splutters or he walks into the dealership the guy is the first to jump out of his seat and come and look after my dad or my mum. The fact the guy was a salesman 15yrs ago and is now dealer principle helps though 😉
Sometimes when you are getting on a bit service is as valuable as price.