On average
60% of delays attributed to NR of which 30% are caused by external factors
20% of delays are external factors
20% of delays down to Toc in question
For Virgin its lower:
During period 5 Network Rail was directly responsible for 50% of delays to Virgin trains with a further 20% down to external factors (such as fatalities). Virgin itself was responsible for 16% of delays with other operators responsible for 14%
Im impressed Virgin, East Coast, TPE etc have the performance they do considering journey length, naturally they rely on some recovery time being built in. Look at Merseyrail, average service length something like just an hour but only 73.0% Right time punctuality, Virgin and East Coast have service lengths of many hours traveling much greater distances and exposing them to a much greater risk of delay, but not much lower ratio of less than a minute late.
Edit: I would love to see a ratio of delay minutes to km travelled, that could be very telling for directly comparing tocs.