C4 Grand Picasso woes

[BMRuss]

Have a 2011 C4 Grand Picasso, been a lovely thing for a year, but now encountering a strange problem.

Flashed up on the dash that it needed a new battery, so installed one I did (was a PITA of a job!).

When I put the key in the ignition after putting the new battery in, it flashed up Electronic anti theft fault, car would just crank over but not start, full power to everything with the new battery, so I guess an immobiliser problem.

My key fob (my wife's car so I have the spare) wouldn't lock / unlock the car, but her key would, my key starts the car, but hers doesn't!

Then last night I went to unlock it with her key and nothing and nothing with mine, just the handbrake light would come on inside the car, then it would work again 20 minutes later.

Is this a case of going to a main dealer once I can get in it and start it and have the keys re-programmed or a new set of keys?

All from changing the battery, French electronics aaarrggghhhhhhhhhhhhhhh

[BMRuss]

I just tried to start it straight away, didn't know I had to do anything else?

[BMRuss]

It has got the little fuse box on top yes, I've checked that and all seems to be fine.

I'm interested in this reinitialising business as I've not heard of that before so any further info would be gratefully appreciated

[BMRuss]

Thank you very much for the above, I shall try this later and report back.

In the meantime, my wife's key no longer remotely locks or unlocks the car, but now does start it, even with the 'electronic anti theft faulty' message, weird!

[BMRuss]

Tried the above but didn't work, but thanks for taking the time to post it.

Looks like a trip to Citroen is in order!