well, CBS kindly exchanged my vented cap for a non-vented one at no cost other than postage and the smell from around the filler has unsurprisingly now gone. Unfortunately the general smell in the garage has not
It's been well over a week since I swapped the filler and many more since I last spilled any fuel (a month) so unlikely to be residual stink from that. The smell is strongest from the back end of the car so I'm starting to wonder if it is the facet pump itself that is defective but no real evidence to support that.
Running out of ideas now as there's no sign of a leak anywhere