ok, so i know border collies are bred to herd, chase etc....but any suggestions on the following would be great!
i have 6 dogs, the one that causes me hassle is a mid-level bitch, she is not a top dog in anyway at all with any of my own, but when out im struggling to think of new ways to stop her chasing other dogs. nothing in particular will spark it off, sometimes she will go , others she wont bother! we dont often meet the same dogs, and after having a gobful from a woman the other day ive started to have enough of trying to ignore it! the daft thing about it is, she isnt aggresive with it, she just chases, and i describe it as a body 'slam' a kind of charge at other dogs, if they run she chases, if they hold their ground she either challenges again or backs off. all of the others ignore other dogs and pass without a problem, my youngster occasionally will say hello but thats all, so i dont feel as if she is protecting her 'pack' so to speak.
it has started to make my walks a nightmare, as im putting her on the lead now and getting her to do down stays etc while people pass with their dogs. but it dosnt seem to help. she is a fit healthy 4 year old agility dog, mad about toys, and very full on with everything, i tried distracting with toys and it worked for maybe 3 walks, treats arent high value enough for her unless we are training, and she is a toad cos she knows the difference!   im considering trying a spray collar, but i know she may not react to it, noise distraction didnt work, we tried a few various things, which stopped the initial chase from a distance, but not as the dog approaches. ( she is the same now on and off lead)

so what im asking is....anything else i could try rather than a headcollar and lead?!

I have a Golden who will chase cats, squirrels and anything that moves (which include plastic bags blowing in the breeze) If she goes to chase a small dog she will only run so far and then when she realises that it is only a dog then she will stop and cme back. However, her behaviour used to spark her daughter to run with her and of course when two dogs are running together there is no guarantees that they will stop. I was so worried that I bought a remote control spray collar. That did the trick. The spray was just enough to distract her and I could gain control with my voice. Her daughter was always more controllable anyway so just getting control of the mother meant that I had control of both. Obviously there will be dogs that it doesn't work for but for me it was the best money I have ever spent.

When you say you tried distracting her with toys and it worked for 3 walks can you elaborate? Were you just waving the toy around and she got fed up knowing she wouldn't get it? Have you tried a ball thrower and tennis ball and actually throwing it all the time so she's completely obsessed with it? When my youngest collie went through a stage of stalking other dogs I'd throw the tennis ball in the opposite direction from a dog walking towards us - or off to the side so by the time she got back with the ball the dog is past. I still carry the ball thrower on every walk and if I see an on-lead dog approaching that we don't know, all I have to do is lift the ball thrower up to have both collies dancing round in front of me oblivious to everything else. As I'm sure you know already collies are obsessive, yours is currently obsessed with chasing other dogs, can you make her as obsessive with a ball?

I'd think very carefully about the spray collar, it may work, she may ignore it or collies being collies what if she's terrified of it? Ok, she'd stop that behaviour but say you're running an agility course and somebody opens a can of coke ringside and she thinks it's the same noise? Unlikely I suppose but you never know :)

Does this only happen when she's out with the others (if so, is it any particular combination of your dogs), or does it also happen when you're out with her on her own?

the only walking on her own is always on lead, road walks where we don't meet as many dogs, when she is with the others  she mainly follows my old boy, and isnt very toy interactive as she stalks him the whole time! she is quite a thick skinned dog, she DOES however get worried by fireworks, but that is pretty much it.  re the toys on a walk, she loves playing tuggy, and will do so until a dog catches her eye, then forget it, she is gone, very odd for a tug obsessed dog?!

ooo wanted to add, i dont play ball often with them on big walks anymore, as i have had a few injuries, including one with twisted gut recently so still not allowed to run about, so we walk further instead! each dog gets 2 x a week training wether in a class or at home, i am currently clicker training meg at home and now also in class for contacts, maybe i could work this into her behaviour shaping?

I suggest you try some individual offlead walks with just her and not any of the others, to properly assess her behaviour. It might be that she is more responsive without the others, in which case individual walks could well be the longterm solution. However it might make little difference - but you have to try to find out. Yes, I know it means double the amount of walks, but that's always a potential problem in a multiplt-dog household.