My 04 roadster has developed an intermittent problem. When it's been unused for a few days it won't start. Everything works fine, but when you turn the key nothing happens, all warning lights come on as they should just the starter doesn't activate. Leave the car for a while and come back to it, it starts fine. This has happened twice now over the last couple of months. I took it to a garage the first time and they so called put it on diagnostic test and told me the battery could do with replacing, which I did, but obviously that's not the problem. Any ideas