My router was supplied by Plusnet (firmware V1.0.2.30_53.0.66B1). It has several bugs in the port forwarding function.

First the good news:

• It WILL forward port 80 to port 80 on another server

Now the bugs:

• The User Interface prohibits forwarding any inbound port to other well known ports such as 8080
• The User Interface permits forwarding to unknown ports such as 6080
  • but it does not actually forward the traffic from port 80. A connection is permitted to port 80 but there is no traffic on it.
    • but port 6080 inbound which should be blocked is now open and forwarded to 6080 on the target server