Well it's only a heavy wrecker in relative terms - quite light, really.
The way the image is shot I'd go for a Ben Hur one ton trailer behind the crane. If it is another vehicle parked one space further on, it's even easier to identify - half ton Dodge WC pickup, which is the only other thing that would look like that from the same distance.
So, either;
jeep, jeep, jeep, GMC with crane and Ben Hur, then GMC with full canvas
or
jeep, jeep, jeep, GMC with crane, Dodge 1/2 ton WC pickup, GMC with full canvas.
If you look at the relative size of the No 7 set and the truck it is mounted on you see it has to be a GMC CCKW, as any other bigger truck would make the crane look much smaller.
While we are covering the odds, someone is sure to point out that the No 7 set COULD be mounted on a Studebaker US6 or even an International, but given the numbers in service in the Uk at the time it is 90% + likely to be the humble GMC CCKW