The only thing I remember about Mediator Linesman was that from the moment it was installed its value was that which could be reclaimed from it in scrap copper. This from a Sqn Ldr who had been concerned with its development and who was going through a list of defence related computer projects (most of which shared a similar verdict) when I attended an ADP course at Blandford in 1972. As an aside he pointed out that cables feeding it were accessible via a manhole outside the Soviet Embassy...
He quite liked the Army Pay Corps computers though (IBM I think) which worked, unlike others.