:Hi Cilla, surprise surprise...
Your profile says you drive a 2007 Clio so it can't be an R reg can it?
You didn't need to break a leg just because you bought a Clio. Couldn't you have just bent down a bit further instead? As the donkey said, it wasn't just his legs that were too long, it was his ears as well.
From your description it sounds like you have a suspect earth linkage; that's the cable/strap that goes from your battery negative terminal (marked either - or blue) to the chassis. I have read several reports that they tend to corrode. Sometimes the positive goes funny too so they both need to be checked for loose fixings or corrosion. Look for signs of verdigris or loose ends. If you are now getting no response, check that the battery is fitted correctly.
Sad if you bought a battery when you didn't need to.
The immobiliser usually toggles on/off with each keypress but it can be effected by the doors. If it is on and you close the door then press the central locking, it will most likely lock the doors but switch off the immobiliser and vice-versa; this means that you have to synchronise them until the immobiliser only switches on when you click on the central locking.
So it may be that you have two problems: 1] the batterty not connected properly so check that first including the polarity, and 2] try clicking OFF the immobiliser when the doors are open.
The other possibility is that you have water leaking in from the aerial and it has effected the immobiliser circuitry. You need to take it out and dry it and hope it hasn't been damaged permanently.
Altogether, I don't know if you will be able to do this yourself, but I hope so or it might cost a lorra lorra pennies.
Tarraaa and good luck
P.S. I have to qualify that my comments are strictly generic and do not refer to any particular model.