What discount on new cars? - Help please

[All2Chewy]

My parents are looking to buy a cheap run around and have decided upon a Daihatsu Sirion, they have an old banger which qualifies for the scrappage scheme but should they still get a discount on list price over and above the discount provided by the scrappage scheme?
If so how much as a percentage is reasonable?

I know that companies will be keen to see the back of some of their new stock and with them being cash buyers I am hoping they can get a good deal, thank you in advance for your help...