Hi Alison,
The same happened to me recently. I ordered a GRO cert on 24th April for a death, it was despatched on the 28th April (can't complain about that) but it didn't arrive until two days ago (23rd May) in Australia.
Same as you! That month seemed the l-o-n-g-e-s-t month, as I was waiting everyday for the mail to arrive hoping it would be there. As it turned out the cert was not for the correct person, so that was disappointing. I knew when I ordered it that it may not have been for the correct person, as one only has limited information to go by on the FreeBMD site.
Perhaps International Mail services have the go-slows at the moment!