Virgin Media Alert: Your device may have a malware infection

I've just had a generic email with the title "[Important] Virgin Media Alert: Your device may have a malware infection".

"We’ve been notified by a third party that malicious traffic has been detected coming from a device using your internet connection. We don’t know which device this is, but it does mean your personal data and online financial transactions, including credit card purchases, could be at risk."

Is there any way of getting more info on this? Assuming it is legitimate I could do with an idea of what I'm looking for to troubleshoot rather then being given "opportunities" to buy AV software...