Restoring Technicolor TG582N Router Settings Problem

[#1]

I have just been connected to Plusnet's fibre network aka BT Infinity and it all works fine with our SBS2003 LAN once the internal IP address range had been changed to match the server. I then wanted to configure a spare router in case the main router ever failed and found an identical unit on ebay for 99p.  So I saved the configuration file from the working router and then restored the configuration to the backup router so it's ready to roll should the primary router ever fail. 
I then decided to test the backup router on the main LAN to make sure it all worked as expected.  Oh dear. Neither the original password or the password from the source router would permit a login and the backup router was unable to connect to the BT Openreach box.  It's as if the configuration file has corrupted the backup router.  Even turning everything off and back on didn't resolve matters. Has anyone else experienced this problem?
The alternative is to simply connect a Netgear cable router to the Openreach box and not bother with the Technicolor router

[#2]

Try a factory reset* on the router, then try a reload of the settings again.
*Press the reset button on the back wile turning on, hold for about 15 sec.

[#3]

Once you've restored the backup router to a working condition (per Hairy Biker's suggestion), then I would recommend trying to upgrade (or downgrade) the firmware on the backup router to match your main router.
You say the spare one was bought off eBay, so unless this happens to be an identical PlusNet-badged unit, then there's no guarantee that it can be re-flashed with a matching firmware build.
I expect there are others here who can comment based on their experience of flashing different versions of TG582n.
As a last resort you could always set up the spare router manually, although it can be very tedious trying to configure a large number of settings.
That said, this is something I have got used to, in trialling various incarnations of Thomson DSL routers