Brucie,
Looks like you have the Eddie Dow dampers fitted to your forks in which case it is most likely that the leak is where the damper rod passes through the top nut. If this is the cases then fit a copper washer between the damper rod retaining nut and the fork top nut. If the leak is from the top nut itself then the most effective seal would be PTFE tape as used by plumbers. I have Dowe dampers fitted to my forks and I had the same problem, and strange to relate on one side only. It is also possible that the damper retaining nuts are lose and just need nipping up. You may or may not be reassured to know that this is not an uncommon problem especially if you have the aluminium top nuts. The stainless steel top nuts seem to be less prone to this problem. There is incidentally no oil seal fitted to the top of the forks.
John