Before doing any trimming I'd suggest just giving the caps and around the engine a good clean using wd40 and some rags, make sure the ht lead is firmly screwed into the cap then a good dollop of thick grease around the Base of the caps before putting back over the plugs and try again- this time of year it'll be the damp causing these probs, very low mileage to think it'll be deteriation in the leads, but if it still arcs then go ahead with a little trim of the ht leads - only about a cm is needed!
Some say...