The reason (as I understand it) the cost per passenger km is so high is that there was (less of an issue more recently) a lot of one and two coach trains.
If you've got to pay a driver for 70 passengers or 210 passengers, the cost between each passenger is going to be a lot more for those 70 passengers.
In addition, average speeds come to play, an electric train which can max out at 100mph is going to need a driver for less time than a diesel train which can only reach 80mph.
If you assume a 50kph average speed for the slower trains and an average of 75kph for the electric trains you've just added 50% driving time by using the slower trains (whilst that may not mean you need 50% more drivers, it does illustrate that costs are going to be higher).
If there's scope for some journey time improvements (even if that's getting some trains which have better acceleration and/or better loading/unloading times) on the Bee Network rail services, then there's potential for that to also aid with staff numbers.
I don't know the area, but if you have a service which takes 68 minutes and has 22 minute turn around at each end, if that service is currently averaging 51kph then by improving that average speed to 75kph the journey time reduces 47 minutes, if you then had the turn around as 13 minutes then you'd only need 2 diagrams rather than 3 to run an hourly service.
Whilst that's probably too much of an uplift in average journey speed, if you could improve the frequency from 2tph (requires 6 diagrams) to run 3tph at an average speed of 65kph with a 16 minute turn around at each end you'd only need 7 diagrams.
That's quite an uplift in capacity and frequency (+50%) for not a lot of extra rolling stock or staff (+17%).