My 110ps HDi 806 either starts first time or not at all, I..

Hi
My HDi 806 has decided to not start more often than start, it sometimes
starts straight away as normall and drives fine but more often than not
it just spins over and chooses not to start. I thought it might be the
transponder that recognises the key but alas not. The pump seems to be
running. If it starts it seems to always start untill the key is
removed and put straight back in then it often wont start, not sure if
that time was just a fluke though and missleading.Anyone have any
ideas, any thing obv. i could check???????

Thanks for looking, cheers

Tim