Any phpBB users? - delayed email problem

VoucherMan

Hoping someone may have an idea, as the person who set up the board is absent, and can't be contacted, and the admin has no ideas.

In case it makes a difference, the board is running version 3.1.

Usually when someone sends me a PM, I get an immediate email notification. Recently though, the messages have been delayed, and I'm not receiving an email until up to 48 hours after the message was sent. I found this old topic, which suggests clicking the "Purge Sessions" button as a solution. I passed this information to the admin, who tried it, but there's no improvement.
The post also suggests reset the cron_lock value to "0", but I've no idea how that is done, so haven't suggested it to him.

So does anyone know how to resolve the issue? Or at least how to reset the cron_lock value? Hopefully the latter can be done from somewhere within the ACP, but not managed to find an answer yet.

Probably worth mentioning too, that I also have a few of the sub forums set to send 'new topic notifications'. These are still arriving straight away, with no delay.