Sounds like the gasket has gone Phill. Not a real problem. If you've got a spanner, try tightening the two nuts up a little......might help. You won't do any real damage, in the short term other than make a bit of soot around the head.
I would try tightening the two nuts, but being a half faired SS that's ridden in all weathers some *ahem* corrosion may have formed and attacking those nuts looks like a job for a professional.
phillc if you do get the nuts changed make sure there either stainless or brass as this will stop them coroding again,i always use copperslip on my nuts but only when the wifes not looking?