OMG!!
you would not believe it. I borrowed the work lead again and pushed it in quite firmly and it started working. I tried both the original and the recently purchased leads but they still wouldn't work. we tried all combinations too to ensure it wasn't anything really daft. I decided it must be the connectors and on really close inspection found that the work one was just over one millimeter longer than the other two. I shaved the plastic off the end of one and now its perfect.
Sorry to have wasted your time.