OK I figured out how to change the refresh rate but the problem is now worse if anything.
The adapter properties for the surface book 2 display (display 1) and the external monitor both state "Intel (R) UHD Graphics 620" as the adapter type - shouldn't they be running on the Nvidia graphics card? I've got the Surface book 2 13.5 inch with the performance base, 16gb ram, intel i7 8th gen with NVIDIA GeForce Discrete Graphics card.
Any help greatly appreciated.