Politely sack him off, fuel consumption is his problem. If he starts whining then sack him off impolitely.
As for Jocks whinging unfortunately it’s become a national pastime for them. I think they see sniping at anyone south of the border as a sport, I guess it’s the Nicola Sturgeon Effect. They used to be great but something has definitely changed in their attitude based on my experiences with them I’d reconstruct Hadrian’s Wall - out of the last five, 1 was great, 2 were arseholes who bought but I’d never want to see again & the other 2 I sent packing within a few minutes of them arriving.
Funnily enough my worst ever customer was a Jock taxi driver from Edinburgh who bought a petrol Octavia estate - the most horrible greedy b****d I’ve ever met who expected the earth & was a cheeky twat. I only persevered with him cos I’d had the car for 6 months, although I did walk away at one point cos I nearly smacked him, his Mrs sort of apologised & said “he’s always like this”. F***ing horrible greedy c*** that still boils my piss whenever I think about him.