Sargent 15778 Solar regulator charging problem

My batteries on my motorhome, both vehicle and 2 x leisure were not being charged by the solar panel and went flat after not being used for four weeks.
I like to have a go at repairing things myself first before spending out unnecessary hard earned cash.
The system consisted of 100W solar panel, Sargent 150W solar regulator, part No155778 and the EC600 Power supply unit.
I always suspected the regulator and after searching loads of forums which went off my topic very quickly and onto something else I decided to try to document my issue with my first video.
I disconnected the solar panel from the solar regulator and measured the voltage and it was above 20V, plugged that back in and then measured the voltage from the solar regulator and this measured 15V.
Measured the voltage from the solar regulator to the leisure batteries and it was 12.6V, which was good and also indicated there was no wiring connection issue.
I then connected it all back up and removed the PSU to measure the voltage at the plug on the E9 on the reverse of the PSU, this was the steady 15V from the solar regulator again indicating no loose connections.
The solar regulator Max output on bulk charge is 14.4V so i suspected that when the PSU detected the 15V it immediately stopped calling for current from the solar regulator and cut out.
Sargent were very helpful when I contacted them via email from their website and after a few exchanges it was recommended I change the regulator but I could not find a direct replacement so Sargent recommended the SX150-12 MPPT Solar Regulator which is 150W 10A.