Just the other day I found out about this prototype. There is not a lot of information available for it and I imagine that very few were made. The consensus is that it was developed as a potential replacement for the L4 Bren LMG. The Cold War era Sterling 7.62mm was also supposedly developed for its ease of construction and low cost compared to the Bren gun and other light machine guns. As WWIII never happened (at least the way everyone planned it) they were never needed.
It is constructed using a Sterling submachine gun trigger group and grip and could use FAL or L4 Bren magazines. In one of the pictures above you can see the weapon with what looks like a proprietary stock. The weapon reportedly used a lever-delayed blowback action to deal with the problem of switching from a pistol caliber blowback action which would be unsafe with the much more powerful 7.62x51mm cartridge. I imagine that it had a great deal of recoil anyway. Others have speculated that the rate of fire would be very high, but that might have been offset by a very heavy bolt or the action of the delayed blowback system. Since the parts all recoil in a straight line, felt recoil might be less than expected, but more in way that it would recoil and be "walked" back down onto the target. I would like to see a modernized version or a version chambered in the cartridge that could have been, the .280 British. I always like finding out about weapons that I've never heard of, even if it was a dead end in weapon design. It is a neat concept and you are not going to see one of these anytime soon.