I have just been chatting to a friend & yet again here’s someone who has been working with a fireman working as an HGV driver on his days off.
Its an HGV’s drivers legal duty to inform their employer if they are doing other work, agencies employ them knowing what they are doing, so plenty of people know, just do not care.
It looks like the best way to avoid tacho law & get away with it is become a fireman.
It is quite legal to work as a fire officer or any other job as long as you take your minimum required weekly break eg 45 or 36 hours.
I used to work in an office mon - fri and do the occasional sat or sun, to earn extra cash.
With the introduction of 24/7 working many people work 4 on 4 off, so they can work 2 days at driving if they want. I knew some shift managers who used to do this, one used to turn up at RDC driving for a competitor 
. I’m not sure what shifts fire officers work but I think it involves rotating days and nights and then a number of days off.
I’ve even know drivers who work mon- fri for one company and do the odd sat or sun for an agency. If it was declared to the first companys employer I don’t know.
Many do other work & remain within tacho law when they drive on days off, but some of these guys take the michael.
Also as there normal work involves driving an HGV another problem is that, as soon as they insert a tacho the rest of that week is driven & worked under tacho law, & that would include work as a fireman.
I Have no problem with anyone working within the law, but these laws are there for road safety reasons.
A Fireman works 3 Day shift on 3 Night shift and 3 off,
shifts are 09:00 - 17:00 and 17:00 - 09:00
So lets put it this way, if they work 3 shifts in one week on night shift thats 3 x 16 = 48 hrs they then have 3 days off which they spend driving in agency work for example, then they go back to work on 3 day shift 3 x 8 = 24 hrs then officially they cannot work the nightshift because they are legally required to take a minimum 36hrs rest after no more than 6 consecutive shifts.
Legally they are oblidged to work under EC Regs because they worked 1 day under tacho.