Hi Wyn!
Thanks for the reply, but there is nothing jammed in the runners. my friend and I have inspected them closely and though we wondered if perhaps little one had dropped a sweetie or a small toy down there: nada.
What I have noticed is that when I pull the bar, nothing moves though there is the same resistance on the bar as there is on the other seats. This is why I wondered if I need to disassemble the clamp mechanisms themselves, though I am reluctant to do this because I don't know how they work and I'm afraid of breaking them completely.
There are small holes in the side of each clamp assembly and I wondered if these were some sort of manual release. I havent poked anything in the holes in case I break something. This is quite frustrating to say the least!
I would concentrate on finding why the seat won't slide, inspect the front handle and bars on either side linking the clamps. I don't think the mechanism is seized, more likely something is broken or an object is lodged in the runners.