Battery Change and now wont start

We have an 03 Micra and the battery was slowly giving up (since a body repair shop let it discharge completely a year or so ago - they didn't know about the extra turn to "lock" with the black button, thought the beeping was normal...!!!) The bad weather hasn't helped and I've had to jump start it twice recently after only a couple of days of non use.
Put a new battery in it and now it wont even turn over. We get a dash full of lights but no starter. Not even the "click" from the starter. The NATS light stays on (not flashing though) but the "KEY" light goes out.The remotes work fine from 4-5m away locking & unlocking the doors. Have undone the battery terminals, left for 15-20 mins then re-attached - still nothing.
Spotted the earth strap from the alternator had broken off (not easy to see under the headlight!) - so replaced that, but still nothing. Even tried a jump start, but that doesn't make any difference.
Headlights come on fine, the blower runs at full chap, so there seems to be power there.
Is there something obvious that I am missing? Do I need to reset NATS in some way? I am thinking that it is a NATS/Immobiliser problem, does that seem sense? Have been tearing my hair out for a couple of days now.
If anyone has an idea, please let me know!!?! I can't believe the simple act of changing a battery over has stopped it working.
Many thanks in advance,
I have 2 keys and some times they work sometimes they dont. If it was the program i believe the fault would be all the time
If you remove the steering cowling you can remove the ignition switch it's a little fiddly then you can use a screwdriver to manually turn

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