Morning, I will be nipping out soon and I'll have a look for a battery for my voltmeter...its a tiny A23 size and hopefully i'll be able to report back later.

Just to clarify....
voltage between 24V- and output one with charge pump active on and off. (Relay and resistor connected or disconnected?)
And voltage across resistor circled, again with charge pump on and off.
Click image for larger version.  Name: Capture.PNG  Views: 2064  Size: 373.1 KB  ID: 26746