I've posted a few times on the question of 'Left Out of Battle' (LOB) over the years. The central question to my mind has been did units rigidly adhere to LOB, no matter what their starting strength was, and did they have an equally dogmatic system for deciding who was designated LOB (as in 1 man per Section, designated 2-in-Cs for Coy and Bn HQs, etc).
I recently got copies of a brief report by the matter produced in 1943. In it the Directorate of Infantry asked overseas commands for their thoughts, with a view to a cohesive War Office approach to the matter. It also commented on the fact that War Establishments did not leave much room for a residue of troops to be left unused.
A draft letter dated September 1943 outlined the general approach for LOBs going forward;
"The cadres to be "Left out of Battle" should consist of experienced and reliable officers, non-commissioned officers and men. The size and exact composition of these cadres, and the length of time for which they are withdrawn, will vary with the conditions prevailing at the time. Nevertheless, the purpose for which such cadres are required is constant and should be clearly understood.
"The main object of having LOB cadres is to ensure that a unit of formation can rapidly become battle worthy again after suffering severe casualties in an action. These cadres should not be earmarked specifically as an assurance against the total extinction of a unit in battle. It will not always be necessary for such cadres to be formed. Commanders-in-Chief will direct their army commanders as to the necessity for having them, and the responsibility for deciding this point will normally be delegated to divisional commanders.
"The size and composition of cadres, where they are considered necessary, will vary with local conditions and the type of fighting likely to be encountered. The arms normally concerned will be Royal Armoured Corps, Artillery, Engineers, Infantry and Reconnaissance Corps.
In estimating their exact composition it will be apparent that there are certain conflicting considerations.
On the one hand it is necessary for units to go into battle at full strength, properly balanced and able to work as a team. On the other hand it is necessary to leave out of battle a correctly balanced nucleus on which to rebuild the unit after the battle. Manpower limitations do not permit any margin on War Establishments from which this nucleus may be drawn. Therefore, in order to meet these conflicting requirements it is necessary to accept a small diminution of strength as an insurance against heavy casualties."
This doesn't sadly offer any insight on what approach was taken by 21 Army Group, especially in light of the infantry casualties that units within it suffered. Other theatres offered the following on their policy re LOB.
Middle East (ie North Africa)
When required – Only on specific occasions, NOT as normal practice. Div comd should make the decision.
Role – Immediate res(erve) who know their units methods and are available to replace cas(ualties) in a few hours. NOT a cadre for forming a new unit.
Should be taken for short (underlined) periods from within WEs of units.
AFHQ (Italy)
When required – no hard and fast rule, Army should lay down policy. Inf LOBs should be detailed when Div deployed. RE (engr) LOBs should only be detailed for specific attack of def(ence). NOT mob(ile) ops.
Take from inf bns, recce regts, RA, armd regts, and fd sqnscoys of C tps. Personnel on courses should be considered LOB. RA LOB should be a balanced cadre.
India
When required – only required on specific occasions when cas likely to be heavy.
Role – Proportion of all ranks and specialists on whom bn can reform in event of heavy cas.
LOB required generally for inf and armd tps only.
The Middle East theatre was the only one so supply a suggested outline of LOB strength for certain unit types;
Inf and Mot Bns
2-in-C
Four or five officers
Two WOs
Three Sjts
Nine NCOs
Forty-seven rank and file
The 2 WOs and 3 Serjeants look to have been from across the full five Coys of a Bn (so 5 in total per Bn). The 47 R&F noted to include 3 signallers, 6 drivers and 2 mortar men.
A couple of quick comments feature as well. RAC note that they have 'spare crews' built into WEs, which allowed them a natural rotation of personnel. RA note they were unable to create an LOB cadre without leaving guns idle so did not feel it was something they could adopt.
Another concern was on where LOBs would be kept and how quickly this would mean they could be brought forward to undertake their replacement role.
So, not much in the way of answers, but would suggest that LOB was not a universally applied practice, and where it was used depended on local conditions.
Gary