Just some feedback on this - I manged to resolve the issue....

It was the service broker...

We send XML messages in our broker and ever since the creation of our broker, we have sent over millions of messages. The problem was that there was an activation proc to Open these conversations, but there was not activation proc to CLOSE these conversations and so the XMLs remained in the tempdb. Once we implemented the proc to close the conversations, the tempdb started to release the space! Cant believe we missed this....

The tempd usage has now dropped to just over 20 GB and remains constant