CG3100D-2BPAUS Cable gateway/router - Port Forwarding problem

I have been using port forwarding for ages with this gateway/router with no problems.  However just recently I created a new forwarding rule with the same name as an existing one.  The router did it without complaint, but something has gone wrong because I cannot now delete the last entry in the forwarding table.  I can delete all the others, but the last one will not go.  I presume I could clear this by returning the router to factory settings but I really don't want to do that - for a start I do not know what the admin password would be reset to, as I do not have a manual.  If anyone has come across this and knows how to get past it without a factory reset, I would be very pleased to hear from you.

The router's hardware version is 2.02.  Its software version is 5.5.7.mp2.V2.06.07.