Hi, just been reading through the thread and I have the exact same problem - intermittently won't start, immobiliser light stays on. Eventually it starts after waggling a few wires but it's driving me nuts getting stuck everywhere!
Been trying to hunt down a wiring diagram with no luck from unhelpful garages, mines a '03 172 Clio Cup - if anyone has one let me know! If nobody replies, I will see if I can buy one off a mechanic and let you all know if I fix it - please do the same! :S
Things tried so far:
*Replaced 2 black relays next to battery (as advised on here, they looked bad when I took them apart on the contacts, worked for 2 wks but back to same issue)
*Checked and cleaned all earths that I could get to.
I've spoked to some garages who had mixed opinions, but none have a clear fix. Some say wiring loom change, some say key code reader change and also someone told me to drop the key on the floor (but its not that as it happens with both keys.)
Dont want to send to a garage as they'll just change things until it's cured - £££!!
If I dont fix it soon I am burning it!