There must actually be stats relating to how much holding there is for arriving flights, mustn't there?
Yes.
The data for Heathrow originates, as you might expect, from ATC (i.e. NATS), but the airport itself publishes a month-by-month summary of average and maximum times spent in the hold in its annual Airspace and Noise Performance reports
here.
The most recent figures I've seen (for 2017) are an average of 4.2 minutes holding per flight over the year and a maximum of around 50 minutes. The 2018 report will be out some time in the Summer.