More than likely the washers under the cover bolts are shot. They are rubber coated soft metal. Over time/heat they compress and don't distribute enough pressure on the gasket.
Be careful in over tightening the cover bolts. They are shouldered and should be torqued to 10 Nm. Over tightening could cause problem.
I'd replace the four washers. They are around $5 EACH. In all probability, the cover gasket is ok, but might require replacement as well.
Mine is leaking on my 2010 and will be getting changed this week on warranty. Triumph do have revised washers for the cam bolts though.
HTH,
H.
