[Geoff Bacon]

Thanks for the hints Mike but they have no effect.

While in the Developer Tools, I looked at the Console output and that showed several errors with the web page; I don't think that is the problem but you might want to check it out sometime.

Reducing the size of the window (i.e. not maximised) removes the problem i.e. looks like something to do with device size responsiveness criteria

Any other ideas?

Geoff