Clonk - Prop Centre Mount?

Question. I have a clonk coming from the middle of the car over bumps in the road. Feels like a heavy mass moving about (I.e. you feel it as much as hear it.) and is worse under power.

I was initaly suspicious of engine / box mounts, but cant find any issues. I'm now suspicious of the prop centre mount (which looks tired). Car has had a box fitted recently, but as far as I can tell there are no issues with the installation.

Anyone out there suffered a similar fate? Are these symptoms relevant to my thoughts?

What kind of job is this to do? I know it means taking the exhasts off (again!!) but does the prop split relatively straigtforward?

Thanks in advance.