As i live in stoke I have used both B.S.Marsons and Platt recently, for parts which had to bee ordered in as these were not kept in stock they were told that all parts were needed to get my car through MOT last week PLATTS no probs parts almost straight away, Marsons told me will take 2 days, as mentioned b4 these parts were for my MOT Retest Friday, Parts were ordered on Monday arrived Wednesday morning, a few more parts were then ordered while I was there Wednesday collecting the parts that had previosly ordered.
I was told would take 2 days to arrived, to which I asked "would this be Friday, if so what time as my car is booked for retest in the afternoon", the sales mans name which i wont mention replied "we have a early delivery friday 8:30am", so I placed and order with them for my remain parts, I was told to give them a call @8:30 which I did, only to be told my parts had not arrived, when i asked what time is the next delivery they said that was the only one, so I contacted PLATTS who said they had my bits in stock, so rushed straight there to pick them up (last ones), after which I rang Marsons to cancel my order only to be told that my parts had "arrived" (surposedly).
So those parts are now sitting at marsons waiting for a new home.
As you mention bartering I always ask for 10% at both PLATTS and Marsons, which I always get.
My preference is with platts everytime.
Also bought a used Brava at platts a few years back now, I managed to get a discount off the brava plus extra for my astra, free bar mats and a few other little things and if I remember correctly I also got half a tank of fuel thrown in too.