Thank you for your advice M. Dias. I wonder, though, if creating a subdomain will really solve the problem. I'm no guru but I don't see the connection. Martin does say that the hostname setting is checked but I don't think that the actual naming of the hostname should have anything to do with it. I'm not really sure if I'm right in saying this.
I got the feeling that it was definitely a hosting problem as after I got a message from netmondo (my hosting company) that they had finished upgrading SQL and PHP everything worked well. (I would have liked to have known that they were going to do that before they started so I could prepare!) In general I'm very happy with netmondo - very cheap with lots of extras. A great place for a hobby page. But I get the feeling that they perhaps are not strong / powerful / able enough to handle the volume of 30 or so students accessing my site at once - particularly when they are all downloading, at the same time, sound / video parts to a class. Any advice on a reasonable but truly powerful host would be appreciated.
I thank you for your interest and hope you don't have any further problems. However just because you put it on a subdomain, I don't think the problem would have been fixed - please, if anyone knows this for sure, I'd like to know! All the best, Eric