I need to instal a y joint in an existing underground section of 100mm PVC drain pipe. How do I do this without digging up a large section to provide the flex to insert it?

Cut out a section of pipe where you need to install the Y fitting. Install the fitting and a short make up piece of pipe. Connect the make up piece of pipe to the original pipe with a shielded metal coupling.

HK gave you excellent advice. Let me explain a bit more. Pick up a 4" PVC wye, a 4" shielded metal coupling, some pvc cement and primer.
Measure from the base of the hub on the wye to the end. Add a 1/4" and that will be your cut. You man now slip the coupling over the bald end of the pipe and prime and cement the wye into place making sure the branch has 1/4" fall. Tighten the coupling and your wye's in place. Good luck, Tom