Thanks, blackbird. I have now fixed that. FWIW I think the thinking behind the standard is flawed, and that its stated aim is probably unachievable - disallowing the target attribute has certainly not achieved it
I made the change and no longer saw the problem. Interestingly, when I undid the change I still didn't see the problem. Unfortunately I didn't think to check immediately before making it so don't know whether, perhaps, the simple act of replacing a file on the server may have triggered something.
I will keep an eye on the situation, and will ask my hosting company whether they have any ideas. Meanwhile I would be interested if anybody knows anything that might have been the cause. If not, I will have to put it down as being a one-off due to some unusual circumstances but that doesn't entirely make me feel comfortable.