has something changed since new year - one for the networking gurus?? RESOLVED

[#1]

Hi All
I have the following setup
Netgear router with internal port IP address 192.168.1.1 and a static route 192.168.2.0 -> 192.168.1.12
Linux proxy server (amongst other things) with 2 cards 192.168.1.12 (I'll call this network 1) and 192.168.2.12 (likewise the network 2) with ip forwarding enabled and no firewalls (currently disabled while I try to figure out whats going on!).
Connected to network 2 are several PCs
I have the following problem
The PCs on network 2 can connect to any machine on network 1 and any machine on network 1 can connect to any machine on network 2 proving that the linux proxy is not blocking anything.
Machines on network 1 can connect to any external service without issues. however machines on network 2 cannot connect to external services.
The machines on network 2 can ping the Netgear router's internal and external addresses but cannot ping any further down the line and  the router can ping  machines on network 2.
I can see packets entering the router when running a ping (mail.plus.net) from network 2 in the router log and see the external link light flash in time with the packets but I see nothing coming back. I can ping the same address from network 1 and get replies.
I am confused, this was working up until the new year break but now does not. Has plus net changed something? or is my router playing up?
Thanks for your help
Steve

[#2]

I'm not sure if it is related to something I investigated today, but the common situation was that it was working l before Christmas but not now and there was a symptom of partial external access from some PCs.
The network is much simpler (eg only one subnet and no router firewall rules).
Netgear router with one PC connected with cable and four connected wirelessly.
All PCs running various versions of Windows and could connect to each other and the network printers.
All PCs could ping external addresses but only the wired PC could browse a website or collect email. On the wireless PCs the browser was being re-directed to a BT wholesale page stating that there was a connection problem.
The router had not been rebooted since 13/11/09 and no odd activity was logged. Also no admin logins had been logged since November (so configuration had not been changed).
No configuration on was set in the router to block external access for the wireless machines.
As a first step to investigating I only disconnected and reconnected the router (ie not reboot or power off/on) from PlusNet.
After that, all PCs were able to browser external sites...
I don't know if the my action helped to resolve the problem or it just "went away".

[#3]

problem solved!!
One update of router firmware and everything started working again, The update even took across all the existing settings so it wasn't a configuration problem as such. bizarre!!!
thanks for the suggestion JohnJ but I had tried just rebooting everything but that did not solve it.
Steve