The Hefah machine gun was a wartime expedient British light machine gun design. It was created by a private company (the Ductile Steel Company) in 1940, in response to a generalized concern that all LMG production in the UK was concentrated at Enfield, and a successful attack on he complex there could create a huge shortage of machine guns. The Hefah design (apparently named for the firm that would eventually manufacture the few examples of the gun made, instead of the designers) appears to have been loosely based on the Lewis, but heavily modified to make it much cheaper and simpler to manufacture. For example, it used a single pivoting locking wedge, in place of the 2-lug rotating bolt of the Lewis.
For magazines, the Hefah used 100-round Bren drum magazines, which latched to the bottom of the receiver. It seems that one of the immediate problems in trials of the Hefah was a serious lack of clearance between the magazine and ground when firing from a bipod, and one can only imagine the clumsiness of magazine changes with such a system (not even considering the rather finicky nature of the Bren drums in the first place).
Information is very scarce on these guns, but it does appear that the MkV version of the gun was adopted formally in 1942 by the Royal Navy, for use as a light AA gun on small vessels. In this type of application, the impracticalities of the design would be minimized, and the simple manufacturing requirements could be met and allow the acquisition of guns otherwise unavailable to the Navy because of other more pressing demands for Brens. The gun was declared obsolete in November 1944, giving it a remarkably short service life. Presumably by 1944 the most urgent concern over LMG production had passed.
A preliminary manual for the Hefah was published in 1943 detailing disassembly and maintenance of the guns, along with showing cutaway views of the action and a dual mounting setup for naval applications. We have a scanned copy of this manual, which you can download here:
A testing report appears to also be available, and we are currently looking into getting a copy of it from a UK official archive.