Thanks for the input.
I also have the 'leave messages on server' ticked as I prefer to delete manually from the website what I don't want.
This is not the problem in my case as in the past the email client has always 'known' which emails have been previously downloaded onto the laptop, and won't download them again. For some reason, and I have not changed anything, it now starts downloading everything.
I have done the obvious things like rebooting, and it has made no difference.