You can park on any yellow lines to load / unload at any time of day or night UNLESS there are these markings on the kerb edge:
http://www.kingston.gov.uk/loading_restriction .jpg
In which case you cannot park to unload during the times stated on the time plate.
You cannot park on pedestrian crossings or the white zig zags and different councils may have their own laws about 'red roads' and so on where it is totally illegal to stop for any reason at certain times.
I haven't heard of a particular law applicable to glass fitters and stonemasons though.
I hope your men are unloading onto a suitable trolley or suchlike.