The river valley near hadleigh has a few small ones and there is plenty of scope for runnign throiugh sticky mud if that turns you on. Try Mersea Island. Not much of a hill, but I recall the embankment around the island might provide some opportunities to run up and down -up and down twice in 100m?
You might need to contact the local cross country officer or even the orienteering chaps to find unpleasant surprises in a small area.
Colchester compared with Aldershot is a poor second cousin I'm afraid - I got so desperate at one point that I spent evenings doing reps on the hill in the town centre that the Army Careers Office is on! Train down to Brecon has to be the only way!!