If an employee works for two or more employers, then the weekly working time is the
combined total of the hours worked (excluding breaks, rest and periods of availability) for all
the employers. The mobile worker must tell their employer(s) in writing, of any time worked
for another employer
ROG:
Do you have a link that states that any holiday time must be added IF the driver works that day as well 
I could not find such rules which govern this situation is why I posed the question
example
Rog you work for me i am your boss i do your WTD you have to by law have 4 week holiday if you work a 5 day week
you must take these holiday now not like before when you did me a favour and i pay you the hours you never took at the end of the year.
so i have to put on you WTD recorded that you had a a weeks holiday which under wtd is 48 hours so if you want to work for someone else i would thing you could do 60 work as it not holiday and it would then be added to you reference point
Rog how many hours do you have to count in a week for WTD would that not be 48 hours for your holiday and any other hours you work for unless the work is not transport related
3.5 Calculating the average when leave is taken
You cannot use statutory annual leave, sick leave, maternity, paternity, adoption or parental
leave in order to reduce the average working time performed during the reference period. So
when calculating the average weekly working time, any maternity, paternity, adoption,
parental leave, sick leave - as well as the statutory paid annual leave entitlement must not
affect the result of your calculation.
the above quote clearly states you still must count your holiday as work
so for 5 weeks the driver has 48 hours work in each week then takes an holiday of 1 week which is 48 hours and for the next 11 week does 48 hours each week, meets WTD 816 hours work
so driver works for 5 week at 40 hours each week takes 1 weeks holiday which is counted as 48 hours for WTD the next 11 weeks does 48 hours would give the driver 40 hour left to do with as he see fit so say he was on holiday in week 6 there nothing stopping him from doing the extra work that week but must tell his employer that he as worked and the he as done what is required for is part if though he does extra hours in the 11 weeks and say he was told by is boss that on week 16 your not going to have enough hour to do 48 then he could go home on a none holiday day IE POA
and you must remember its an average of 17 weeks and as long as at the end of your refernce point you have only done 817 hour your OK