Stuart if you think the Mercedes deal is good then go for it, you aren’t tied into taking anything from Motability… the Government give you a cash allowance to spend how you like.
I had cars on the scheme back in the early 90’s and left cause I didn’t like what was being offered, now I’m back on it (since 2014) cause it suits me, but if it didn’t I’d be off ?
In this situation you’re the boss, you can visit (or phone, email) every garage in the country looking for a deal, you can use your allowance in lots of different ways to get a vehicle and a deal that suits, you can keep the cash and never go out… get prescriptions, groceries etc all delivered to your door