I finally decided to sort out the many "Recording Failed Due To No Signal" issues by fitting a variable attenuator at the back of the 5000T to knock the 99% signal strength down to around 75%. If too much signal is a problem for the tuners why don't Humax fix this in the hardware/software to regulate it down, this sort of problem must be a nightmare for non-technical customers who just expect to plug and play.
Whilst fitting the attenuator I noticed even a slight movement of the existing RF IN cable could cause signal loss so I purchased some Premium Lindy Antenna Cables, which are also shielded, and they wobbled in the connector so no improvement. In the end I have made up my own cables using WF100 Coax and some spare ends which seem a better fit but are not perfect.
The recording and picture breakup issues seem to be fixed but I'm not entirely happy with the stability of the RF IN Cable so I'm wondering if there are any suggestions for better cables to fix the wobble? Just as an aside I did notice when diagnosing the connection that the RF In corner of the 5000T was very warm so could that also be a factor?