By the time the Magnette was launched, Victor Derrington was already a well established engine tuner, who had built up his business on the back of motor cycle racing. When the B-series engine was designed as successor to a previous smaller Austin design, it had been given a cast iron cylinder head with siamesed ports leading to valves that were positioned in line with the engine, not across it.. The induction and exhaust ports emerged from the head on the same side of the engine, so the fuel and exhaust gases effectively did a U-turn in the combustion chamber. Derrington realised that a design that offered a purer cross-flow arrangement would provide more power. Cross-flow means that the fuel/air mixture enters the combustion chamber on one side of the cylinder head and exits on the other. On overhead cam engines, the valves can be arranged across the engine so that the gas flow is a smoother progression from one side of the engine to the other. In conjunction with the firm HRG, he designed and manufactured an after-market alloy cross-flow cylinder head for owners that wanted to tune for speed. The alloy also improved heat dissipation. This was offered initially for the Magnette but as later versions of the B-series were released in 1600, 1622 and 1800 forms he followed suit by offering his cross-flow head to suit them.

He also offered associated accessories like an exhaust manifold and induction manifolds to suit various carburettors including not only larger SUs but also twin-chokeWebers

Practrical Motorist carried out a road test of a modified car and published a report in their May 1962 edition. The car also had an overdrive, so not all the improvement was attributable to the cylinder head, but top speed was  achieved at 107mph! Torque was also considerably improved, with the ability to accelerate away from 1000rpm in top with no pinking.

Original Derrington parts are offered for sale from time to time but MG specialists Brown and Gammons and Moss Motors both offer a modern version of the design. 

This website gives a good history of Derrington and provides some useful facts and statistics. You can also read more about this modification in "How to Power Tune MGB 4-Cylinder Engines" by Peter Burgess