I'm sorry Andymartin, but I don't know how you can confidently proclaim that "we all know FR24 quotes the actual landing and take off times". I dare say there are some users who don't know that.
Secondly, the OP's comment was that the flight is rarely on time, ranging from 20 minutes to 5.5 hours late. What Dave Reid was correctly pointing out is that a flight that FR24 shows as 20 minutes late very likely departed from the gate on time. In fact, with Heathrow taxi times usually 20-25 minutes, and most industry observers (including the CAA in their punctuality statistics) treating flights that left the gate up to 15 minutes late as an on time departure, if FR24 shows the departure was up to 40 minutes after the scheduled departure time, it was really on time!