Hatfield Forest is owned by the National Trust. The man in charge of NT biological stuff, Dr Stuart Warrington, will no doubt be extremely pleased to hear from anyone who wants to record on NT properties anywhere in Essex. This applies to all groups - but they are crying out for entomologists in particular. The Bishops Stortford Natural History Society have done some work in Hatfield Forest in the past and I have trapped a few times in the marsh (which I think is an EWT Nature Reserve), but there are no recent records. I can provide some old moth data if you want to get in touch with me (see EFC programme for my contact details).
The gates are normally locked at night so you will have to get a key. Be aware that Hatfield Forest Car Park is notoriously listed on "certain web sites" as a venue for night-time activities that you may not be comfortable with. It is not a place to hang around waiting to meet people, unless you subscribe to the same web sites as them (!) so meet elsewhere and go in with the key.
I can't find Stuart's work number at the moment - but he is based at the NT office in Bury St Edmunds. That ought to be easy to find.