I'm not familiar with that exact model, but on similar year 129's here in the US (SL500, SL320) I've found a few fuel leaks in this area. Caused by a loose fuel hose at the fuel filter. I simply replaced the factory "crimp" style clamp with what we in the US call a worm gear style clamp. Possibly in England it's called a "jubilee" style clamp? There is a plastic panel under the car in this location, you'll probably see fuel dripping at a drip hole on the botton of this panel. Remove the panel, it's held on by 3 or 4 plastic 10mm nuts, and you'll be able to find the leak.