Construction
Details about the firing ranges within the training grounds are covered under The Woodland Trust section.
We have no information about erection of the camp other than that regarding the installation of mains electricity. It is possible the Depot was constructed in a similar fashion to the contemporaneous RAF Barkston Heath, 7km away, completed in April 1941 as a Relief Landing Ground for RAF Cranwell accommodating 557 personnel including WAAFs (concrete runways added in 1943, re-opened 1944). George Wimpey & Co Ltd built that airfield using Irish workers who would come for 6 weeks then return home for 1 week. They lodged at farms; locals of Westborough village nearby recall them packing into the Blue Greyhound pub each night. Altogether, Wimpey built 93 aerodromes, factories and army camps during WW2. The wage cost alone of building the average camp was £50,000 (opens pdf) equivalent to £6.4 million pounds in 2016 measured as the labour cost. Arthur Batten (reel 3) describes working as a labourer building an airfield at Hampstead Norris working from 08:00 to 22:00 each day.
The asymmetric layout of the Depot with boundaries on two sides following woods and hedgerows was deliberate to aid concealment from the air.
Figure 1: Luftwaffe map circa 1940
The Luftwaffe had aerial photographs of Grantham (figure 1). The original WW1 Belton Park camp on either side of Five Gates Lane and on Belton Park Golf Course (labelled 'd') had been a potential target for Zeppelin bombers. The golf course closed from 1940 (Grantham Journal 14 June 1940) Alma Wood is red arrowed with the depot to be, immediately to its west (click photo for larger image).
Detailed examination of the Luftwaffe map shows the WW1 Belton Park military railway track bed heading south from the golf course to Grantham and its branch running to the Londonthorpe Stables. There, either parch marks or the surface remains of foundations delineate the WWI Londonthorpe Stables. This area was to become the Depot Motor Transport sheds, Drill Sheds & parade ground.
Figure 2: The German Invasion map for ground forces, Nord Midlands. England Blatt Nr. 6. Militargeographische Objektkarten (Oberkommando der Wehrmacht 1941)
The German Invasion map for ground forces (figure 2) fails to identify the Depot site (click for zoomable image).
Key: red crosses - hospitals, Stahlhelm (German helmet) - Grantham Barracks, red triangle - RAF Spitalgate airfield, violet triangle - electricity transformer station, violet circle 'iron industry', i.e. the armament factories and small violet rectangles - stones, possibly referring to quarries & the ironstone industry. Examining the whole sheet, the principle assets targeted are Sheffield, Stoke on Trent and Manchester.
Placement
Was the Depot placement a deliberate attempt to avoid aerial location despite the name "Belton Park"? The Luftwaffe could easily identify Belton House and the Park from the aerial photograph, but taken between 1939 and 1941, before construction of the Depot. That the Luftwaffe did not have regular updates of ground features is revealed by their daylight attack on the Derby Rolls-Royce factory in July 1942. Flying south of Grantham, a Dornier bomber was surprised by an airfield at Harlaxton along with its defensive ack-ack fire. Nearby WWI RAF Harlaxton had re-opened in April 1942.
Figure 3: electricity plan for the Depot, sheet 2 of 3.
Facilities seen on electricity plan
Figure 3 is sheet 2 of 3 on linen of the electricity plan for the camp dated 1943 from the Lincolnshire Archives (the other 2 maps are missing). It provides descriptions of the buildings in greater detail. The medical reception centre catered for only 20 inpatients, but nearby No.4 RAF Hospital Rauceby offered an additional ~400 beds (RAF operated from June 1940 to July 1947).
Located within the medical reception centre is a decontamination unit. The Geneva Gas Protocol of 1925 (both Britain and Germany were signatories) outlawed use of gas in wartime, but not its production. In the event of a gas attack or accidental exposure, personnel would receive first-aid treatment and get decontaminated in this building. They were designed to deal with the gases developed during World War I, lachrymatory agents; respiratory agents and blister agents.
There may have been plans to make use of Belton House itself for military purposes, for in February 1941 Henleys Telegraph Company Limited laid an 11,000 volt underground supply cable 2'-6" deep to the Stables. Power sufficient to supply a small factory. A number of shadow factories were set up for BMARCO. In July 2018, conservation of the Stables revealed steps leading down to a brick-lined tunnel located immediately to the south of the west entrance and leading out under the wall towards the north driveway (figures 4 & 5). This is likely a utility tunnel constructed to take that electric cable. If the same, then Belton was supplied with a WWII cable entering via the Adventure Playground until very recently.
Figures 4 & 5 20th century steps lead to a tunnel in the Stables filled with broken garden urns among other detritus. Archaeologists abandoned further excavation because of risk of collapse of the driveway.