TD5 oil pump bolt help

Hello new to the site...... I've just encountered and overcome the starter motor 'nut of doom' which took me 4 hours to get off and 10 mins to put back on! Anyway 2 days later oil light came on solid took it to a garage to have it pressure tested and was told that there is no oil pressure.....got it back home (literally 50m away) and has been stood outside for 2 month..... It was running fine no nasty noises in fact in sounded better than it ever has done!!?? So yesterday I removed the sump to get at the pump and found that the bolt was loose so therefore I tried to remove it to order a pre loctited one but the bolt just kept spinning and wouldn't come out although a couple of turns the other way would nip it enough to go tight enough to turn the sprocket and hear a hiss from the pump which I presume is air out of the pickup pipe??? Is it possible for the bolt to still be in but not drive sprocket?? I'm thinkin that the bolt has sheared half in the threads but that's just a guess..... Anybody else had any similar problems??? Thanks chaps any info would be great.