You state that you bought the car in May 2011 - I assume that's a typo and should be May 2016, as you say 5 months ago.

Under the Consumer Rights Act, any faults that happen within 6 months of purchase are assumed to have been 'present or developing' at the time of sale, and the selling dealership is entirely responsible for the repair unless they can prove otherwise (see below). This is nothing to do with the warranty.

However ... the 'see below' bit ... if the problem was caused by 'bad fuel', then the fault would be down to the petrol station, and you'd need to show the fault was due to the fuel (getting the fuel tested at your cost), and then sue / threaten to sue the petrol station for the cost of getting the car repaired.

So, 2 different approaches you could take.

1. Go back to selling garage, quoting the Consumer rights Act, and demanding a fix. They may well bluster, but they'll have to knuckle down eventually, just get Trading Standards involved (or threaten to). Ideally, communicate via email / post, so things are on record.

2. Get the fuel tested (no idea what the cost would be to you for that), and then depending on the results you either go to the selling garage as in (1), or the petrol station, to get it fixed.

Do note that under the CRA, whilst you have the right to the repair, you do NOT have the right to have it repaired at a garage of your choosing (so you might well have to get it taken up to Cambs, unless they agreed to let a local garage do the repair), and any costs for such transportation and re-delivery would be YOUR responsibility, and not recoverable.

The filling station will not have had the wrong fuel in their tanks, be assured of that. If they had then lots of other people would have the same problem as you and there would be hell to pay.

Personally I would not trust any main dealer telling me I needed new injectors, and independents even less. Very rarely are any of them diesel fuel injection experts. I would go to a good diesel fuel injection specialist who will have all the right equipment to test and repair injectors. There are plenty of them around.

The OP sets out that the above vehicle has a petrol engine......

Oh yeah, so he does. Doh

In that case I definitely don't believe it needs new injectors. What brings the dealer to this conclusion I wonder? Petrol fuel injector electrical faults are very rare, and if the nozzles were partially blocked then removal and specialist cleaning should cure the problem. The only likely contaminant that could be present in the fuel is water. This will of course cause the engine to run very badly, or not at all, but the chances of long term damage caused so quickly is almost nil. Draining and refilling the tank and changing the fuel filter will most likely cure the problem.