An update to this thrilling saga. In September last year, the gas had once again disappeared. The stealer supposedly fixed again but, whaddya know, all gone again this February. Having given up on the very poor Lancaster Tonbridge (now Jardines) and because after 2,000 miles the FF needed a service (lockdown, sigh) I tried out the new Guy Salmon mega shop in Sunbury.

They actually listened to me. They could not trace the leak BUT said they would discount the cost of the gas and the work by 50% AND if it leaks again in the next few months will refund me what I did pay. Seemed fair to me. I continue to have A/C anxiety, though!

They often inject a dye in with the gas, if it leaks out it's then very obvious where it escaped. I had a pipe on an Audi that leaked but only at certain times/conditions, the dye made it so there was no doubt. BMW i3 Electric Car
2012 Full Fat RR 4.4 TDV8 (now gone)
2006 VW Touareg 3.0 TDi V6

Yes - the Tonbridge mob said they injected dye, but not apparent. Guy Salmon at least sounded more confident about their dye!