JB-007@work

Hi , i have owned a 55reg , A8 Diesel 4.2tdi for almost 3 yrs with no issues Around a year ago the car failed to start one morning (just a faint click) I tried it every day for almost a week until i returned from work one evening and tried again & surprise ! It started ! I let it warm up then on , off started it approx 10 times. It has been fine until it done the same a few weeks ago. Once again i tried it for around a week again & surprise , it has started up again !!! Any ideas please