Welcome Nick (yes you guys, I am still occasionally on here...)!
If you are looking to get the best out of Epping Forest there are a couple of places you could start depending on your preferred ride time/day and how you like to ride.
Epping Forest Epics are a good, mixed bunch (age & sex) who ride from just off the A406 at the A104 Junction on Sat & Sun mornings from about 10am. They ride obscure (but some very good) singletrack up to High Beach (about 12 miles actual riding distance), have a grub stop and then head back another way to get back about 2pm (later in the summer when they'll ride further). Used to ride regularly with them when lived in North West London and always found them friendly and helpful.
Epping Hard Cru are a younger bunch (if you ignore Old Man Steve!) who tend to ride earlier (8am -ish) from High Beach and usually head more North and out from there so cover a different area to the Epics. Again, a friendly bunch who probably cover less distance but try a little more technical stuff (or just have a giggle!). BTW, they are not 'hard' in the real sense, the name is more than a little tongue-in-cheek (well Steve could probably rip you limb from limb if he wanted but age has mellowed the beast within him - jolly nice chap, actually!)
Quite a few people ride with both these groups, they get on and it's not like being married, you can ride with others!
Hope that this helps!
G