I have the above from Post Office Broadband. After several attempts trying to update the 2009 firmware. I got the following reply from Zyxel support
"If the current firmware on your P660HW-T1v2 router is 3.40(BGQ.3), you cannot upload the 340APR5C0.bin firmware file and vice-versa.
There are no later firmware updates available after the 3.40(BGQ.3)"
It seems Post Office do not want people updating it, despite there being newer firmware out there for this router. Please also see thread
»UK Firmware for 661H-D1
which I wanted to add to, but now closed.
Hopefully this of use to someone. If anyone has found a workaround for this particular problem then let me know.