Webtrak ? Is that still going ? Have you tried the later Aircraft trackers such as Casper or Planefinder.net, they have replays up to a year.
If you're looking for accurate arrival and departure tracks for 100% of LHR movements, neither Casper nor PlaneFinder will give you that. They can't track aircraft that don't have ADS-B (bmi Embraers, for example) and they can't accurately track aircraft with inertial-based navigation systems such as Lufthansa B735s.
WebTrak, on the other hand, captures 99.9% of movements. It also displays accurate heights, which the aforementioned systems don't.