Elitebook 840 G5 dock integration/compatibility

[Cladach]

A while ago my work issued personal laptops and then more recently docking stations and dual monitors to replace the old fixed desktop systems which were getting very old. The laptops were good (mine is an Elitebook G5) but the introduction of the docking stations (HP USB-C G5s) were a revelation and as I was working from home a lot I bought myself another HP G5 dock and a pair of displayport monitors to emulate the set-up I have at work. So far so really good. Switch on is by pressing the power button on the dock and never a problem.

So good was this set up a decided to buy myself a newer laptop too and thinking I could simply use the same dock and dual monitor solution the sensible thing was to buy another laptop that used USB-C and the simplest (I knew it all worked after-all) was to buy another HP. I bought a used one (it's also an Elitebook 840 G5) from an online site which I guess has been wiped and new OS etc installed? Although it very happily talks to the  HP Support Assistant and has downloaded software updates - I cannot say for definite that it's got a completely original set of HP software it would have been delivered with from new?

My problem is that whilst the laptop is great but it doesn't play happily with the dock, like my work one does. It will power on from the dock power button but it's then a battle to get it to display on the monitors. Sometimes it will all work but most times I have to unplug the USB cable, get signed-in and tell it to display on another monitor. The keyboard and mouse seem to freeze whilst the USB-C cable is plugged in so it's a juggle of clicking the mouse as you plug in and it often takes a few tries to succeed. Once it's all working a restart of the laptop is often successful to reconnect without problems.

I've tried updating the BIOS, video drivers and most of the other software I could find via the HP Support assistant and from the HP support website, I tried removing and allowing windows to re-install the USB driver, as recommended on another support request on this forum but I'm not convinced I'm made any difference. What am I missing and any suggestions to fix it?

Last thing to mention, my laptop reports it's model number as 3RU09EC this doesn't seem to appear in the compatibility list for the G5 dock (or the G2 dock) is this my problem and is it fixable, or am I doomed to have to fight to make it all work and be extra frustrated when my (seemingly identical work laptop) works so simply and reliably?

Many thanks for your help

Richard