I Know from first hand experience that the Employer CANNOT under any circumstances force a driver to reduce his / her rest period.
I recieved a call one afternoon whilst in Harlow from a TDG Assistant Transport Manager insisting I be back at Newton Aycliffe for 7am 
I still had 2 deliveries to do which would take me to 5pm the journey back was something like 5 1\2 hrs and I needed my rest, I was told to start heading back and take a 9hr reduction to which I refused, I was advised that if I did not get back for 7 am, things could be pretty bad as the company I was contracted to may ask for me to be taken off the contract 
My Reply simply went something like, Well you’ll have to find another contract to put me on then as I wont be back for 7am 
Sarcastically he replied You Will !!!
Anyway, when I arrived back at 9.30am he was not a happy bunny and was full of threats about my employment with TDG, I simply took out my mobile phone and called Graham Bird the URTU Regional Rep, the assistant TM’s face was a picture when I stood in front of him telling Graham the story
A Meeting was held with top brass where it was pointed out that the Drivers legislation actually states “A Driver May Reduce his daily rest period to 9 hrs on no more than 3 occassions”
There is absolutely nothing in the legislation which permits an employer to reduce the drivers rest period without his / her consent 