Domain not renewed!

[#1]

A friend of mine who has his domain hosted with PlusNet and gets all his mail delivered to it, suddenly discovered that he was getting no mail. I investigated and found his PN mail address worked fine, it was just his domain that didn't work. I now see that the domain was allowed to expire despite PlusNet supposedly having to auto renew it.  It seems another host has grabbed the domain.
What to do?
I presume PlusNet are liable for purchasing the domain back from whoever has grabbed it?

[#2]

Can you let me know your friends account details so we can take a look?

[#3]

Chris thanks for your help on this one.

[#4]

OK, Have got the real story now - apparently the domain was not registered with PN, but with Nildram (as was), now TalkTalk Business. They never sent out a renewal notice email AFAIKS.
Perhaps we will have some luck with the "Redemption Grace Period" in getting the domain back from the Domain Sniper?
This is all a bit of a nightmare, as his email is going to a third party potentially.

[#5]

Sorry I couldn't be more help, good luck with getting the domain back.