Let me say first, that I am a very new rider but I have seen massive gains in doing similar to the following.
@
nickg I see your in Colchester. I'm in Ipswich and know that finding a good training hill will be nearly impossible. But try this; warm up by riding a steady pace for about 20 minutes. Then head to St Andrew's Ave - hit it quite fast and climb it for 1.5km -
http://www.strava.com/segments/2300514
Turn around and now head down Cowdray Ave, you'll get a good amount of time to recover. Now around the roundabout, past the Institute and smash Balkerne Hill as hard as you can but go in slow and increase speed as long as you can, turn around, recover down the hill and do it again. Continue with some distance or go home. Rest the next day then do it again. This time reversing the method of attack; go into St Andrew's slow and increase speed / power. Then head to Balerne Hill and go into it as fast as you can and try and hold as much speed as possible - head back down the hill and repeat. As before, continue with ride or go home.
If you do this twice a week and once a month seek out something longer, Gun Hill near Dedham isn't far from you, and you'll start to see some big changes to the way you can handle climbs within a month.