Firstly eggit2 I reckon you're really lucky - the trails looks great fun to ride and have clearly been constructed with many hours of loving care, props to the diggers.
Now I'm a digger too, and have been the main, but by no means only, at our trails for approx 10 years. In that time I've met people who hold every opinion I've seen voiced here from welcoming trail hippie to ardent trail Nazi. Our situation is slightly different to most - our trails are council sanctioned and in a public park which means having a diggers only policy just isn't on the cards and personally I wouldn't want it even if it was. However that doesn't mean that I think its completely open season on who gets to use the trails - in the same way I get annoyed if the bmx kids go and trash the adjacent football pitches I won't stand by and let wanton trail destruction occur at the hands of scooter riders/small children who think they're climbing frames/riders with no manners. The argument that just because other people don't ask to ride street spots means you shouldn't show respect to trail builders just doesn't hold up - two wrongs and all that??
Anyway the point is from the look of those pictures that spot is nowhere too public, now obviously I could be wrong about that and fair do's if I am, but personally I would be more than happy for respectful trail riders to ride on the basis that they had the means and ability to fix what they broke. Actually what is slightly less righteous to my mind is then taking photos and posting them on a very well known website. For anyone who is doing there best to keep their trails on the down low the worst thing you can do is advertise their presence here. You might argue you can't tell where they are from the pics but believe me the minute pictures get out people ask questions: they know where you're from, they realise its an old spot they used to ride years back, some little kid pipes up they know where they are etc etc and before you know it your nice little project that had escaped the radar is no longer hush hush. That wouldn't matter nearly so much if along with the influx of riders and hangers on and rubbish, it didn't attract attention from councils and landowners and busybodies all of whom are liable to have your years of hard work flattened as soon as look at them.
As an aside I always try to be welcoming to new comers, old or young, bmx or mtb and know that for every 50 I meet that come and have fun and ride on the dusty sunny days when all's to be done is water, sweep and ride, I'll get one who will be keeping me company on wet, cold February weekends when the real work gets done. As others have pointed out ettiqutte is just manners and common sense - if you arrive at someone else's trails, however public, and the locals are digging go and offer a hand. If you aren't local I certainly wouldn't expect you to give up your whole day but just show willing. It has to be said its the ones that grate are the locals that ride more than I do in the summer but somehow just don't have any free time in the winter, not because they're busy on their own set that I know I have an open invitation to at any time, but because they're sessioning the skatepark in every gap in the showers and the Playstation in between. But c'est la vie.
That's my two penn'orth – well actually a darn sight more but I should be working and this was a welcome distraction – apologies for the essay!