Thanks Brogan, as ever, for the prompt and helpful response.
I did, my server team ensured the forum was 'vanilla', exactly was it was (minus the upgrade) in the test import. I've just dropped all tables and currently uploading the backup. Hoping advice on here will reduce a second failure.
I did check retain IDs, perhaps I should try without retaining? Is there any implications to that?
[Edit] The private messages actually looked to have imported fine on the forum, after it hit 100% though I couldn't continue to actual import to any other area.
Retaining IDs is used for when you want all imported content (members, threads, etc.) to keep the same IDs as the forum which you are importing from.
The destination forum needs to be blank for that to be possible though.