Neil, I'm afraid that the Vanderveen book doesn't include any background info but the IWM photo reference is something to go on. They should be able to provide you with a decent copy together with whatever caption was given at the time.
The 'H' series relates to the British Army in the United Kingdom commencing in the late 1930s.
I've searched the online archives and H 11484 isn't there but H 11489 is and shows a battery of 5.5" guns so probably taken at the same time. It is captioned as 240th Battery, 51 Medium Regiment RA at Ellesmere Port, July 1941.
http://www.iwm.org.uk//collections/i...ject/205197627
The Arm of Service serial '5' with Corps bar above is shown in the Vehicle Markings, 1941 listings as 'Medium Regt.(How).' but of course doesn't indicate which corps formation sign would have applied here.
51 Medium Regt. were 3 Corps troops during the 1940 campaign in France but I don't know where they were allocated after returning.
Rich