KenG,
It sounds to me like you have all the info you need to be able to set a trap to catch the thieves, i.e., typical time of the day that it occurs and the day of the week that it typically occurs. You could probably even buy an inexpensive bike (BSO) to use as bait. I'm surprised that the thieves struck a 2nd time to take another bike, which makes me think that they're stupid enough to try a 3rd time.
Perhaps your neighbors would be concerned enough to participate in capturing the thieves, or perhaps a nice quiet Retrobike party with some RB members that are local to you would make for a good solution. They could arrive early on a Saturday or Sunday (Noon or 1 PM?) and in groups so that there aren't a noticeable amount of cars on your street, and everyone could have a nice quiet afternoon dinner inside, away from the windows (or with curtains drawn) so that the thieves don't know that so many people are home, and, assuming you mount a covert camera in the shed, you could have a large monitor available for all inside to see if anyone takes the "bait". Just be sure to take the thieves down fast and call the authorities right away. I'd even video the entire capture event if I were you, both for proof of the crime as well as proof against any claims of violence on your part. Even if nothing happens, it might be a good opportunity to get to know any RBers that live near you; just a social event with food, drink, and like-minded bike-loving individuals.
I know it sounds like a lot of effort, but I'd certainly volunteer to be part of the "posse" if there were enough participants (Obviously you'd want to be sure that your posse outnumbers the thieves, which is apparently a group of 3, so that there's little danger of you or anyone in the posse getting seriously injured), and of course if I lived in the UK and within a reasonable distance from you.
I know you're probably thinking that it's easy for me to say that when I live on the other side of the planet from you, but my perception of the UK (and, to all, please pardon my ignorance if I'm wrong) is that firearms are quite rare on your side of the pond, and therefore I'd think that a large contingent of fully grown men could most probably capture at least one of a group of 3 teenagers and turn him over to the authorities.
It's a different situation entirely in the US, both for thieves and homeowners because you never know who has a gun. I live in a rural area where nobody dares burglarizing a house unless they thought absolutely no-one was home because they might get shot, and I wouldn't dare approach a group of teenagers in the US, even as part of a large posse, for the same reason. In a land where firearms are rare, however, I'd think there would be safety in numbers, although I'd have a billy club handy just in case I'd need it to defend myself.
You obviously need to think that there's a possibility that there may be repercussions if you escalate the situation, and of course you'd never want to put your family in danger, but sadly I suspect that the current situation will continue until the thieves are captured.
One more thought: Because she's apparently the same approximate age as the thieving scum, I wonder if your daughter might actually know any of them, perhaps from school? She may be a valuable source of information should you decide to launch your own covert investigation of some possible suspects. She may not know them personally, but could potentially give you a list of suspects, especially if she's ever noticed a group of 3 individuals that habitually congregated and who seemed to be bicycle-oriented. If it were me, I'd think it would be great to discover my property in the possession of a couple of prime suspects because then at least I'd know exactly who I was dealing with and could then direct my attention specifically to those individuals, as opposed to trying to defend myself against the unknown.
I wish there was more I could do for you than just dispense advice. If it helps, I'd be happy to pay for the beer if you do host such a party of RBers, just so long as everyone toasts me with the first drink and speaks fondly of me (after that, you can say whatever you want about me

). That said, I think I'll be in deep financial trouble if more than a couple dozen beer-drinking RBers show up at the party :shock:

, so perhaps I should set a cap of $100 towards beer. Just post pics of the event here and I'll gladly "Paypal Gift" you.
Best of luck to you, and please keep us posted if your bikes turn up; I think we'd all love to hear a happy ending to this story.
Cheers,
Frank