is there a gas vent on the slurry tank?

there is. (fluid end, gas relief , hidden pipe block, fluid end)

Had issues with this and also with a lack of flow from the distillation ports and pumps on the refinery side of things too, even when using the Devs prefabricated drilling rig and experimenting with adding the new valves 'etc'.

I've got my fingers crossed that all it needs is a quick hotfix and that it's nothing more serious.

I can duplicate this bug. I was building my first oil rig right as the patch happened and have been trying to figure out why it doesn't work. Slurry will pump through a drill swivel when it is not connected but once the swivel connects to a drill rod/drill head slurry bugs out in the swivel. Rapidly connecting/disconnecting the drill swivel will cause a small amount of slurry to go down the rod and you will drill... Pretty sure with vehicle damage on this would cause the thing to explode. Slurry is still moving into and out of the drill swivel but it is not going into the well and no saturated slurry comes out.

I have tried a million combinations of fluid/gas relief valves, pumps, pressure sensors ect. It will not go into the well.

Thank you for the report. This has been fixed.