Sounds like you have given yourself a tough gig to start with Sparkie!
If you can, try and hunt somewhere easier, just so that you can get familiar with your machine first i.e a beach or around some play equipment. In the long run, it will reduce your learning frustrations.
If you are hell bent on this particular site though, you are going to need to do some experimenting. When you get the overload signals, try and note, which notch that they appear in. Eliminate these notches. If this doesn't help, the only 3 things that you can do, is lower you sensitivity, try and raise your coil off the ground a bit or try and hunt the spot with the small coil.
But like I said in my opening comments, this particular site, is probably one of the worst spots that you could pick to learn the hobby in and will lead to considerable frustration! So if you could be patient with yourself for a bit, try and learn in an easier environment. The site that you described won't be going anywhere, and has probably not seen too much detector action, due to it being such a tough spot to detect. So take consolation, that somebody else will not hunt it out on you in the mean time.
Good luck with the hobby, and keep those questions coming.
Mick Evans.