I've just been going through the logs and I can see the majority is a single user but there are other users also getting them. Non of the connections are over the wifi. I'm just looking into if the downloads are to the same device, although i'm almost certain they are different devices as one of the users is a teacher and would only use her machine or a staff room machine and another user (majority user) is a 6th form student who would not have access to either of her machines.
I think the idea of it being an app downloading ads is a great one but doesnt add up in our case as the downloads are going to workstations that are locked down.
I'm going to try a report that spans a much longer time period and see if I can spot any kind of pattern on who is receiving these and when.
I've blocked them for now, so we'll see if the "scream test" yields any results.