Abslolutely without predjudice or taking any particular stance on the matter.....

As CaptainZok says, a single track road is only that - a road wide enough for the roads engineer to deem it suitable for only one 'vehicle' at a time and has provided passing places so that two can pass.
If there was a weight/height restriction on the same road then that would be signposted in addition and stipulated at a suitable place to allow any overweight/oversize vehicle to take an alternate route.
As the vehicle driver, it is
your responsibility to assess whether any road or part of a road is suitable - or safe - for you to take
your vehicle down or past - it belongs to Comet, but in your care for the timebeing. If that means getting out the vehicle and walking the route to judge suitable, so be it. If you decide at the outset 'I'm not going down there' then head home, simple. Item undelivered. If you decide its safe and, partway, have a change of mind then unfortunately, its up to you to get yourself out of it again. If, in the process, you collide with and damage something well, unfortunately, so be it. It was your choice to go there at the outset....
If you go down the road, make the delivery and safely get out again, brill!! You have obviously done better than others have done. You must therefore ensure that you have the proof to show your vehicle is undamaged when it was in your care. As stated, use Comet's paperwork and systems, if you can. A line manager would have signed an undamaged vehicle out to you and another accepted it back in again. Provided you did not pass a rickety wall at warp speed such that wind gusts blew it down - well, it must have been unsafe in the first instance....
Best of luck.
B