Google search results for Royal navy MDU bring up Reseaching WW2.co.uk, Abbreviations and Acronyms. MDU is listed as Military Dispersal Unit.
www.researchingww2.co.ukAcronym Finder is also in the search results. And has MDU as Mine Disposal Unit for Military and government. But no information if this relates to WW2 or the present day.
I have no idea if Military Dispersal Unit is correct or not and if that is what it was. Some information below from Norfolk Heritage Explorer (Google search)
Philip L
Record Details
NHER Number: 38934
Type of record: Monument
Name: Site of possible World War Two Royal Navy 'Y' service station east of Middle Street
Summary
A possible military site dating to World War Two is visible as standing structures on aerial photographs taken in 1942. The site comprises up to two buildings and four posts, towers or masts. There is no obvious agricultural explanation for the structures, although at least one of the buildings might have had an agricultural use. Given the amount of military activity in the area during World War Two a military function seems plausible. The site is located approximately 500m to the west of RAF Trimingham (NHER 6799) which was established in 1941 and used as a Coast Defence/Chain Home Low and Oboe radar station during World War Two. Given the proximity and the nature of the site described here, some function associated with radar seems possible. It may even be the Royal Navy 'Y' station which monitored German radio traffic and was reportedly sited at Trimingham. In the absence of any supporting evidence, however, the site remains enigmatic. One of the buildings at the site is visible on aerial photographs taken in September 1940; the remainder on photographs taken in June 1942. The site had been entirely removed by June 1946.
And from RAF Trimingham, Norfolk, report by Wayne D Cocroft (Google search)
Also at Trimingham, from May 1941, but apparently not near the Radar Station, was a
Royal Navy "1' service station. Its function was to monitor German radio traffic and by
means of direction finding aerials try to locate the source of the transmissions (Foynes
1994, 227).