Thanks for reply.

So is the routine not able to cope with such large offsets? I only ask as I saw some videos of people printing on really slanty beds (1cm+) just to prove it worked.

As I said in my previous post, it was nearly level originally but I was still getting the same results in the same places as I am now. I was actually getting better results at the time if I just did a G28 and printed without doing a G29 at all, so seems a little odd.
Thanks!