Thank you so much to everybody for their suggestions.

The next day after posting the problem had stopped. It has not , since then, recurred with the same virulence, although I got a couple of cheeps last night, which makes me think that perhaps it was in fact cell-phone interference as suggested by Graham above.

Weatherbug is a possibility, and if the problem recurs I will investigate if this is the reason.

The noise did come from the speakers btw, and I have a broadband connection so I don't think it could be an automatic dialler ?

Once again, many thanks to you all for taking the time to post your advice.