Rover 75 2.5 V6 Starting Problems

[Stagman42]

Hi I'm new to the site. Am on my 5th Rover now. Had 4 45's never had any problems with any of them other than one being written off as a result of an accident. Always loved them as I can work on them myself. Reliable and cheap as chips to run. Never had any head gasket problems either.
Now on a Rover 75 2.5 V6. It's an absolute gem of a car so comfortable and effortless to drive. Just one problem every now and again it will not start. When you turn the key just get a clicking noise. This always seems to be worse when it has not been run for a few days or so. When being used regularly is ok. Have been told it could be the cam shaft sensor, crank shaft sensor, starter motor or immobiliser problem. Battery not been tested but lights do not dim so assume ok? Has anyone any ideas of what to try? I’m leaning to the battery as when it is run around should be charged up and so that’s why it works. However, I have paperwork to say the battery was changed in 2011 so it is getting on a bit. I am assuming if it was one of the sensors that the engine would turn but not start is this correct? So battery or starter motor is the question. Any help or advice would be greatly appreciated. Thanks Pete