The subcontractor is refusing to return to site and as a critical trade is causing delay to the project. I need to mitigate this by terminating their employment and engaging another subcontractor to carry the work.
You need to give them notice under clause 7.4.1 specifying the default or defaults which in your case would be either:
Clause 220.127.116.11 without reasonable cause wholly or substantially suspends the carrying out of the Sub-Contract Works; or
Clause 18.104.22.168 fails to proceed regularly and diligently with the Sub-Contract Works.
You need to make sure that any notices are sent by some form of registered post.