Yesterday, after filling up with fuel, there was a noticable whiff of petrol while driving along. There followed a forensic inspection of all the usual places leaks may occur: hoses around injectors; hoses behind engine; hoses on fuel tank link pipe; fuel tank brackets; etc. In the end, I noticed signs of fuel weeping from where the filler pipe meets the left hand fuel tank. It appears that the weld has cracked where it meets the tank.
In retrospect, this is not too surprising because we know that the tanks move up and down and we also know that aluminium is fairly brittle and doesn't flex too well.
My plan is to seal the area with fuel compatible PU sealant but I couldn't help wondering if a flexible section of pipe in the region shown might have prevented the problem in the first place.
My car has covered just over 10,000 miles. It's possible that the crack may have been there for some time and I've only just noticed it. I did give the car a particularly generous fuel fill on this occasion!