On the 8th February 1915 Capt. P. S. Cookson (late Royal Sussex Regiment) was appointed to command the Battalion with the temporary rank of Lieut.-Col. with Major W. H. Hamilton as second in command, and Major E. V. Wellby as Adjutant. The company commanders were Capts. A. A. N. Hayne, S. W. J. Limpenny, E. D. Wilson and A. E. Wood. The training facilities which had been extended to the 2/4th Battalion by local residents at Barnet were accorded to the 3/4th Battalion also, and the training of recruits under company arrangements proceeded as rapidly as possible and as efficiently as the circumstances permitted. No time indeed was to be lost for the 1/4th Londons were now in France, and as already described began to suffer battle casualties early in March 1915; so that it was clear that the 3/4th Battalion might at any time be called upon to make up its deficiencies. Towards the end of April it was in fact called on to supply the first reinforcement, and accordingly despatched 2 officers and 50 other ranks who, as referred to in Chapter IV, joined the 1/4th Londons in the Ypres Salient. Third line battalions had also been formed by the other regiments of the 1st London Brigade, and hitherto these had been distributed over a wide area on the outskirts of London; but in the last week of April the four new battalions were concentrated under canvas at Tadworth (Surrey) as the 3/1st London Brigade, under the command of Col. H. C. Cholmondely, C.B. The 3/4th Battalion joined the Brigade on the 26th April. On the 5th May the Adjutancy was taken over by Capt. E. E. Spicer with Lieut S. H. Stedman as Assistant Adjutant, Major E. V. Wellby having transferred to the 1st London Regiment. Early in June 1915 a general reconstruction was effected in the reserve and training cadres at home; and a number of fresh battalions were formed composed largely of personnel who were not medically fit to serve overseas. To this end a composite Battalion, known as the 100th Provisional Battalion, was formed of officers, N.C.O.'s and men of the 3/1st London Brigade. On the 2nd June Capt. A. E. Wood, and 2/Lieuts. E. J. Bennet and J. S. B. Gathergood and about 100 N.C.O.'s and men left the 3/4th Battalion at Tadworth to join the 100th Provisional Battalion which was stationed at Aldeburgh. On the same day Major W. H. Hamilton was appointed to raise, equip and train a new Battalion, to be known as the 4/4th London Regiment. Concurrently with this development the 3/1st London Brigade moved from Tadworth to billets at Bury St Edmunds. Here training was proceeded with, and the battalions of the Brigade were again opened for recruiting to make good the gaps in their ranks caused by the formation of the Provisional Battalion.