I have suffered for the last 12 months with the same old Clio problem, jump in the car one day and it starts fine, another time turn the key and the immobiliser light will not stop flashing, therefore no point in turning the engine over and draining the battery, the problem was you did not know when this problem was going to occur, sometimes I had to sit in the car for up to 15mins turning the ignition on and off until the light remained solid red, then it would always start 1st time, that was the infuriating part about it.
Tried everything I read on this forum, change battery keys, etc, etc to no avail, then I happened to mention this problem to a neighbour who had just purchased a s/h Clio for his daughter that had suffered from the same problem as mine and obviously many others, he cured it at a stroke by purchasing a Renault Immobiliser Bypass device that just clips into the engine management plug (situated under the ashtray, no tools needed) this was purchased from www.remotekey.co.uk
, I phoned them up and spoke to the owner, who could have not been more helpfull, all he needed was for me to undo my ignition key and give him the number printed inside, this I did at 14:00 on a Monday and the item was with me by 10:00 Tuesday Morning as he sends everything out by special delivery, it took me 1 min to fit, and HEY PRESTO!! I had a Clio that now starts everytime with ease, the total cost was £84, this was money more than well spent, I have now had it for 7 weeks with not one non start problem.
The bypass plug can be taken out with ease, as will have to be for a service to be undertaken, and just popped back in when its been done, when I ordered this item he told me that this was the 10th sale of the day for Renault Clio's alone!!.
I hope this bit of advice will help someone else out there with this major problem.