Fraud is a police matter. Give them a call and let us know how you get on.
I made a claim 2 years ago, and won got full money back that was against SSE. I think in this case it's pretty straightforward.
Like buying a car that does 150mph top speed, but when you take delivery it only does 130mph because the manufacturer has fitted a restrictor ... and then they say if you take it off you invalidate your warranty.
Well ... it's not just me using the 12.4l/min value. Glow worm themselves use it for all their other boilers. All distributors use it. And all installers use it.
But ... the boiler actually only does 10l/min.
That's fraud.