I emailed Humax last night asking about the issue and also asked about any firmware that might be upcoming, i got the following response.
Hi JamesThank you for contacting Humax, I'm sorry to hear about the trouble you're experiencing with your Humax equipment. We are aware of the C4 recording issue, this has been passed on, to find the source of the issue and have it resolved, we do apologize, you will however be able to record from C4+1 for the meantime, also there is a new firmware update being formed, though no timescale of when it will be released.
Best Regards,
Colin
Humax Customer Support