First, thank you for your replies and advice.
As with all things IT related, I've found it's better to wait a while before replying.
An update therefore.
Earlier this week, a full banner page for a porn video appeared, and was blocked, by Norton.
Two days later, another banner ad for a "War games" site appeared. This had no obscene content and was passed, as it were, by Norton.
Yesterday, I received a banner ad saying my Flash Adobe was out of date and to upgrade now...which I didn't.
I have carried out a malware check and nothing has come back.
I would also say, and this is purely for the info for any site techies please, that may be following this, that, it is only on this site that I get such ads.