If it really is just a matter of over-oiling and the plugs are in fairly good nick then to get sparking again the plug(s) just neads degreasing.
Make a visit to Maplins, Radio Spares or simular and buy an aerosol can of electronics degreaser. The cans are small enough to go in a pocket or toolbag, it should come with a length of thin plastic tube to fit the nozzle that will go right into the plug. The solvent leaves no residue, is pretty well inert to plastics & insulation and evaporates quickly.
A quick squirt, a quick shake, screw the plug back in and you should be on your way. A proper plug clean when you get home might be in order but please avoid abrasives, you never know. Use a magneto file on the electrodes or if unavailable a ground down flat swiss file.
Also excellent for de-gunging points, cleaning bearings and all sorts of little jobs, bike and domestic.
