Ah, apologies for this one, it was caused by a mod apparently.
Although I did have a separate issue that I'll make a report and bug for I think, often times before impact my TGP shifts visually to an offset point maybe 20-30 meters away from the target.
If I slew the tgp it returns to the correct target position, and laser guided munitions also go to the correct position, so I think it's just a visual isse.