Raspberry Pi server on port 81

[#1]

I have a PC running a web server on port 80, and static IP address. Everything works fine. I thought I would try running a Raspberry Pi server as well, but am having some trouble.
The Pi has Apache running on port 80 and I can access the internet fine. I can see the server if I type 'localhost' into the Pi web browser.
I configured the router (Technicolor TG582n FTTC ) to translate port 81 to 80 for the Pi server. I thought that in the outside world I would be able to see the Pi web server if I typed <ipaddress>:81 but it's not responding. My firewall is set to 'low'. I've tried having the Pi on cable or wifi, no difference.
Any assistance gratefully received.
Cheers
Steve

[#2]

Instead of mapping the port in the router, set the server to use port 81 instead, perhaps?

[#3]

Hi,
Have you configured Apache to listen on the interface bound to your LAN?

[#4]

can you see the server from inside the lan?
i.e. <pi ip> in a browser?
If not then check the server is listening on 0.0.0.0 not 127.0.0.1

[#5]

What you're trying to do does work, I've done exactly that in the past including the raspberry pi bit -- but different web server.
If it helps these are the port forward / port translation settings I used to configure the TG582n router.
http://npr.me.uk/nprmeuk/pf/
Just checked and the plusnet firewall does not block port 81 when on the low setting. Don't know about port 1080 as used in the above link.
Have you double checked the firewall is set to low?

[#6]

Thanks for your help, everyone. In fact I had done everything right but the router must have got all confused. I reset it to factory defaults, reassigned the ports and it all worked! I just have to connect my temperature probes and my freezer will be on the net!