S Type auto selector prob

Hi all,I wonder if anyone out there can give me some advice,I picked up an older (2000) S Type a week ago,3.0 v6 SE auto,after a few days the ugly got stuck in P problem arose,managed to get it out,now to start the ol girl,,,,,ignition on,footbrake and then drop it into N and off we go,when stopped and shutting down it will go back into park but wont start (although it has a couple of times)and when ignition switched on when in P,the P lights up for a second then goes out...So really two questions arise(1)I am assumiing that the selector is playing up,it seemes "out of adjustment"is the best way to discribe it,I have seen an aftermarket aluminium "fix"to replace the plastic bits in the selector,has anyone done this one and is it a simple (ish) job, ....and(2)Using the start procedure I have come up with(for now)any ideas if the rest of the box will play up in the same way until can be rectified properly?One more thing........it appeares that a second hand selector was fitted by  previous owner at some point in the past,,Thanks for any advice  ............Rio