I did come across a company called Douglas Concrete who were formally Douglas Concrete and Aggregates Ltd from 1962-1998. There was also Sir Robert Douglas who had a plc and many other companies and was heavily involved in construction and civil engineering amongst other things (see the two links below). His activity seems to be mainly earlier though. I don’t know if there is any relation between Douglas Concrete and Sir Robert Douglas.
https://books.google.co.uk/books?id=9Hr ... td&f=false
http://www.heraldscotland.com/news/1212 ... t_Douglas/
I couldn't see any tippers, but I came across a few Douglas concrete trucks (see link below) but again I’ve no idea if these are related. The Douglas name appears in black though, but white on the cab fronts, but these trucks are later than the mb tippers. So this is all just a bit of idle speculation but it was quite interesting reading.
http://www.na3t.org/road?search=douglas
Slightly off topic, there are many references to Pointer Transport and part of their business was the movement of aggregates, cement, etc. If you scroll down this link, you can see a concrete truck from the Pointer group with the Pointer logo on the door that we all know.
http://www.trucknetuk.com/phpBB/viewtop ... 6&start=90