Some more information on the cartridges found. The range in dates and manufactures is quite fascinating as is presented below:
Defence Industries, Verdun, Canada - two cartridges (both 1944 and both with a nitro-cellulose propellant - prefix "z" - both fired by a rifle); head stamp DI z
South African Mint, Pretoria, S.A - one cartridge fired by a rifle (date 1943); head stamp U
ICI, Kynoch, Yeading, Hayes, Middlesex - one cartridge fired by a MG (date 1941); head stamp K4
Crompton Parkinson Ltd, Doncaster, Yorks - one cartridge fired by a MG (date 1942); head stamp C-P
Kynoch & Co, Witton, Birmingham - two cartridges (one fired by a MG, date 1940 and one fired by a rifle, date 1939); head stamp K
Royal Laboratory, Woolwich Arsenal, Kent - three cartridges (one fired by a MG, date 1939 and two fired by rifles, dates 1928 and 1929); head stamp R L
Crompton Parkinson Ltd, Guiseley, Yorks - three cartridges (one fired by a MG, date 1942 and two by a rifle, dates both 1941); head stamp CP
Royal Ordanance Factory, Radway Green, Cheshire - two cartridges (one fired by a MG, date 1941 and one by a rifle, date 1941); head stamp is WO arrow
Greenwood and Batley, Leeds, UK - two cartridges (one fired by MG, date 1941 and one by a rifle, date 1941); head stamp GB
Winchester Repeating Arms Co, New Haven, Conn., USA - one cartridge fired by a MG, date 1942; head stamp WRA
Dominion Cartridge Co, Canada, one cartridge fired by a MG, date 1941; head stamp DC
In addition were four cartridges fired by MG where no date/head stamp could be read and also four fired by a rifle.
I also found one .300" cartridge fired by a rifle (would have been USA 1917 model or USA Springfield) - manufactured by Dominican Cartridge Co, Montreal, Qubec, Canada (head stamp R), date 1941.
So if my maths is right 29 cartridges, 12 factories, two calibres and dates from 1928 - 1944. Just to the south, where the Sizewell Power station now stands was a rifle range (often referred to in War Diaries), so this area was certainly in use for training as well as defence right through the war. Finally I also found a complete clip of five 303's (although the charger clip had all but rotted away in the acidic sandy soil).
Top photo - difference between a rifle (right hand) and MG (left hand) fired cartridge
Second down - .300" cartridge - note lack of a rim.
Third down - clip of five 303's - note the cordite propellant clearly showing in the damaged cartridges
Bottom - a clip of 303's in their charger clip (WW1 vintage found on the Somme - any policemen reading this the bullets are not live!!! - fired cartridges with fired bullets pushed back in) and the German Mauser charger clip for comparison.