Despite having deleted all bar the first page of messages in my PM inbox, I am still getting a PMs Full message and cannot send any further PMs.

The screen says I still have 477 messages out of an allowable storage of 100, even though I have deleted all bar my 50 most recent messages. I'm inclined to think there is an error and that I needn't have deleted the messages in the first place ...

Can anyone help?

 

that's cause your membership has expired.

either keep deleting or renew your s2ki membership

 

... I didn't think to check

 

You might have messages in Sent box and other folders. Check them too.