MacBook Pro 2017 daisy-chaining with DisplayPort I have a 2017 13" MacBook Pro (with two USB-C ports) with Mojave and wish to use it to daisy-chain a display to two external monitors through DisplayPort.

Unfortunately, it seems that although the hardware supports this, MacOS itself does not support multi-stream transport - the two monitors in the chain are mirroring their display with no option to extend instead.

Is there any third party driver or other software available to make this work as intended in MacOS? If not, what would be the best alternate solution to get it working? Ideally, I'd like to avoid buying a separate £200 dock to be able to use multiple monitors and still power the MacBook as well.

The monitors in question are Dell U2515h if that helps - they support daisy chaining through DisplayPort 1.2, although ideally, I'd also like to be able to chain to another monitor too.
No, there's no third party driver or other software available to make this work. You say this is the way it was intended to work, but as far as Apple's documentation goes, it is not intended to work this way on macOS. MST is simply not supported, and you'll need to use Thunderbolt 3 for display chaining.
You do not need a 200 pound dock in order to use multiple monitors and power the MacBook. Various smaller and larger docks in different price ranges exist for Thunderbolt 3. Usually they have a combination of a Thunderbolt 3 port and a DisplayPort or HDMI port for connecting two monitors simultaneously.