It's probably a clock input. The small signal you're seeing could be just the CLK picking up other signals on the PCB.
Scroll down on this link to "External Clocks".
That link is really useful. I understand now why a clocked signal capture could be really important if the capture rate isn't fast enough to resolve different logic state changes. However it may be, for the work I'll do, this won't be a problem.
External triggering seems to be useful though as everything can happen so fast you may not capture what you are looking for or capture it and not be able to subsequently find it among the mass or collected data especially if you don't use a protocol that decodes it into something more readable.
Will try some experiments later when I get back to the workbench. Anyway I won't be buying a very expensive LA for my hobby fiddling about and I'll make this one work.
Purchased a hand held logic probe and this is also proved useful and I now have a logic probe I plug into the breadboard which I hope will perform well enough.
Cheers again for the info link, very good article.