Anything wrong with using MTB saddle on hybrid bike I'm been having problems with fit on my hybrid. 
When I spend around 1hr on the saddle I start to get pain in the left prostate.

I can no longer ignore the problem because I plan to do C2C. Right now I need to do some training because I'm not that much better than a couch potato. I have a feeling I won't make it otherwise.

The thing is I have two other mountain bikes and I don't seem to get the same problem.  

Shall I just get a MTB sadddle?
This is fine. There is not really any hard "Hybrid" and "MTB" saddle categories. The  cycling gods will not smite you down for committing bicycle components miscegenation.