I've been banging mine on the counter top for years. Nary a problem. It does of course leak when inverted, but not terribly. You get used to it. It's more of a dribble. I bang mine on a piece of soft pine wood that is set on the counter top. Yep, a washrag is part of the developing protocol. I use two washrags myself, and the tank rests on one the rags, which is placed on the wood. The other rag is for wiping up.
There are probably dozens, if not hundreds, of good patching materials available to fix your problem. Simon is certainly being very nice and professional about this though. If it were my tank, I'd go up to the hardware store w/ the tank and see what sort of glue/caulk they suggest to patch it, and be using the tank tomorrow. There's epoxy, and silicone caulk, at my home, and I'd feel confident using either. This stuff below works on just about anything. I've used it frequently to repair motorcycle fuel tank leaks. If it's impervious to gasoline, oil, and diesel, it will be fine w/ developing chemicals, especially during the brief time they're in there. Devcon is great stuff too, and that would probably be my first choice, although I think it's a little bit overkill for a plastic tank leak.
If you can clearly see the crack, and before you patch it, use a drill or a sharp knife to make a shallow impression (not all the way through!) on either end of the crack in order to keep the crack from spreading.
http://www.itwconsumer.com/versachem-products/product.cfm?id=Heavy-Duty Fuel Tank Repair Kit-150