Can't Change TCP Port Using Configuration Manager For X64 SQL 2005 SP1 Virtual Instance

Hi, I hope someone can help me!   I changed the TCP port that the default instance of SQL 2005 x64 SP1 is listening on and now SQL server won't start as the port is being used - no problem I say to myself, I'll just change it again, however I can't!  

My problem is that I can not change the TCP Port back again using Configuration Manager - no matter what values I put in, after a restart of the instance it comes up with this in the IP Addresses tab for TCP/IP Properties and SQL Server won't start:

IP1
Active   Yes
Enabled   No
IP Address  aaa.bbb.ccc.111
TCP Dynamic Ports
TCP Port  0

IP2
Active   Yes
Enabled   No
IP Address  aaa.bbb.ccc.222
TCP Dynamic Ports
TCP Port  0

IP3
Active   Yes
Enabled   No
IP Address  aaa.bbb.ccc.333
TCP Dynamic Ports
TCP Port  0

IP4
Active   Yes
Enabled   No
IP Address  10.1.1.1
TCP Dynamic Ports
TCP Port  0

IP5
Active   Yes
Enabled   No
IP Address  127.0.0.1
TCP Dynamic Ports
TCP Port  0

IPALL
TCP Dynamic Ports 2020
TCP Port  2020

Interestingly, the ip address that the server is listening on aaa.bbb.ccc.444 is not reported in the list of IP addresses in the configuration manager for this instance, yet it is when I view the available IP addresses for the other two instances - is this significant?

I have tried failing to the other node, changing it there, changing it at the console and also while logged onto the the virtual machine remote desktop, rebooting both nodes, etc, etc.   There are two other instances on this cluster that are still working fine but I hvae not attempted to change the TCP ports they listen on.

I used the same method to change this default instance as I did for a named instance on a similarly configured cluster and it worked fine.   How to fix this?

Regards, Mike

2007-02-27 12:03:17.69 Server      Server is listening on [ aaa.bbb.ccc.444<ipv4> 2020].
2007-02-27 12:03:17.71 spid9s      Starting up database 'tempdb'.
2007-02-27 12:03:17.72 Server      Error: 26023, Severity: 16, State: 1.
2007-02-27 12:03:17.72 Server      Server TCP provider failed to listen on [ aaa.bbb.ccc.444<ipv4> 2020]. Tcp port is already in use.
2007-02-27 12:03:17.75 Server      Error: 17182, Severity: 16, State: 1.
2007-02-27 12:03:17.75 Server      TDSSNIClient initialization failed with error 0x2740, status code 0xa.
2007-02-27 12:03:17.75 Server      Error: 17182, Severity: 16, State: 1.
2007-02-27 12:03:17.75 Server      TDSSNIClient initialization failed with error 0x2740, status code 0x1.
2007-02-27 12:03:17.75 Server      Error: 17826, Severity: 18, State: 3.
2007-02-27 12:03:17.75 Server      Could not start the network library because of an internal error in the network library. To determine the cause, review the errors immediately preceding this one in the error log.
2007-02-27 12:03:17.77 Server      Error: 17120, Severity: 16, State: 1.
2007-02-27 12:03:17.77 Server      SQL Server could not spawn FRunCM thread. Check the SQL Server error log and the Windows event logs for information about possible related problems.