If I change wheels from 38 offset to 43mm, does it mean i need shorter bolts? if so what size?

Offset has no bearing on wheel bolt length, are they genuine or aftermarket wheel?

I have oem wheels off a mark 4 golf (which are 38 offset, 5x100) and I want to replace them with oem fabia wheels (which are 43mm) and pcd 5x100), does this make a difference? My existing bolts can fit both wheels? what about the locking wheel bolt..i can keep that as well?

Edited 9 October, 201114 yr by fabia55

Yup they'll be fine

that's great. thanks, I assume I can use the same locking bolts as well.

Yes you can.