Matt, if you stay ‘on site’ during a working day and do not drive on a road open to the public then you do not need to use the tachograph as you will be exempt from the EU drivers hours rules. You will also be exempt the UK domestic rules so you don’t need to keep log books either.
On the days where you drive on the public road between the two sites (even if it is only 200 yards) or when you pop down to the filling station (a mile away) you now come under the EU drivers hours rules. Tachograph cards / charts must be used and you must obey the daily and weekly rest requirements and limits for continuous, daily, weekly and fortnightly driving times. The activity you describe, collecting chemical waste, doesn’t fall under any of the exemptions or derogations from the EU regulations. You only fall under the Road Transport (working Time) Regulations if you drive (under EU rules) on 11 or more occasions in a reference period of less than 26 weeks, or drive on 16 or more occasions where the reference period is 26 weeks (or more). Drivers who are governed by the Domestic Hours rules do not fall under the control of the RT(WT)R. What does this mean? if you do 16 or more trips on the road in a six month period then you will be subject to the RT(WT) Regulation.
Regardless of how many times you drive on the road, you come under the EU drivers hours rules so it would be very advisable for your employers to get some tacho charts issued and drivers to get and lean how to use digital tacho cards. Company also needs to get some downloading kit and start complying with the relevant legislation. If you let me know where you’re based, I can arrange for my ‘firm’ to visit yours and explain the fine (no pun intended) detail to them 
The fact that the vehicle is not laden when it travels on the road makes no difference as far as these rules are concerned.
Tricky D,
if the vehicles you use have a permitted weight (including that of the trailer, if they draw one)
of 7.5 tonnes or less and they stay within 50 km radius of base and you are employed as a machine driver or hole digger rather than a driver and you are carrying the machine on the vehicle for you to use, rather than just delivering it, then you are exempt EU drivers hours rules and can drive under the Domestic Hours rules. However, be careful as you have to be able to prove all four parts of the derogation to claim it. The bit about driving not being your main employment basically means; if you lost your ability to drive, would you still be employable by the company as a machine operator, hole digger, scaffolder etc?