If it's a cable operated clutch (all mk1's, not sure about mk2), check that the cable isn't rubbing where it passes out and below the firewall/ bulkhead. Mine had rubbed the plastic covering away allowing water to get in and make it rust leading to a very heavy clutch pedal. I removed the cable and squirted WD40 into it from both ends and worked it back and forth until it was free again.
Another thing that could be causing problems is the release arm that goes into the bellhousing. Mine had partially seized, only discovered when I changed the gearbox for another.
Other than that, there is not much else you can do for a heavy clutch other than changing the clutch itself.