I have contacted Nochex and guess what....they have sent me back to this thread
Reading through the full 13 pages again I have found the following statements.
If anyone can confirm they are true?

The Nochex APC version embedded in Zencart 1.3.8 download does NOT work.
(You can take payment but cannot tell who has bought what)

Nochex does work with both merchant AND seller account

It is possible to remove the Nochex APC from Zencart 1.3.8 and then install the latest Nochex APC from the download section (Which works with Zencart 1.3.7)

Any help would be really appreciated as the last thing I want to do is start paying £20 a month for CC payments. I would rather donate £20 to anyone on here who can help instead.