OK, I've now 'got round' the header text display problem. I had disabled the display of the tweety bird image, which was obviously acting as some sort of a placeholder. I've now reinstated it as a transparent png and all is well.

Another problem is, though - the filtering of '@' is too aggressive. It doesn't just filter '@' in position 1 - it filters it anywhere. How do I change this, please, so the code only filters tweets sent directly to one user...but it still allows tweets that mention a user?

Thanks.