Aylesbury is a fine and very knowledgable dealership in my experience although if you turn up to collect your car during that awful after work period you'll have to wait your 15-25 mins turn in the que with no seat to park your tired ****..
I choose Bedford as it's easier to get to from Milton Keynes but if I was Leighton Buzzard based I would choose Aylesbury.
Note also that ASM Autorecycling are based just around the back of the Renault Dealership.. Worth a try if you do need any parts for an older shape Lag II or Lag I for that matter ppl..
Can't comment confidently on the reliability of the newer shape models but do beleive there have been significant improvement on the design of the
charger, intercooler system which has been criticized in the earlier versions for poor reliability. I also believe the newer versions have shorter service intervals ( was previously every 18k miles ) which you would have to weigh up in terms of cost of ownership..
As for the reliability of the earlier versions my petrol 1.8 16v now has 115k on the clock and still returns a very reliable 40mpg on long journeys if I keep it under 70 mph..I would try a petrol for a weekend if your dealer will let you..