The mucky bikes you see at the weekends are likely just green-laners out terrorising the bridleways, these are the people you need to get to know if you catch the bug!
For organised days out you need to find something like the group I went with a couple of times (Clews Leisure), they were great but sadly no longer in business. The only other peeps i know of is one i don't have any experience of, but here's the link if you want a look...
http://www.yamaha-schools.co.uk/ they have days going on in Winchester which you might be interested in.
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