Ok i recently got a later a12 motor because i buggered my old one up... when i polished it up i found a hole that was blocked up.. i looked on my old motor and its a straight through hole... i know its a oil return or water maybe but my question is can i leave it there or do i have to drill it out.. or why is it there in the first place.. cheers

You do not need to drill out this hole.

On all the A series engines the oil for the rocker assembly is delivered via the middle head bolt hole. On the early engines the oil passage you are looking at goes out the top of the block, into the head and then makes a 90 degree turn into the bolt hole. On the later model A series engines the oil passage makes the entry into the bolt hole in the block instead of the head.

If you look in the side of the block just below where you think the missing hole is you will see a small plug in the side of the block. This is where they drilled the oil passage into the middle head bolt hole.