Hey Ian, the first thing that I would do is check your timing and listen to your engine at idle, then rev it by pulling on the throttle cable and listen to see if you can hear where it is coming from.
I am guessing that your valves are ticking, so you might just want to pull the valve cover and take the 10 minutes to check the clearances.
Let me know if you need anymore help.
Do you think it could be a fuel problem?? Octane maybe?? Does it sound like it is pinging??
It could be a bad tank of gas or a low octane level (if your running high compression).
Just go put some new gas in and see if that helps. I doubt it will since your not running a higher compression.
Other than that I cant think of what else it could be without looking at it myself.
Good luck with the fix, whatever it may be.