Looking for a new job should I mention that I won't do overtime work
While interviewing, you should ask questions to learn about the company culture and to determine if the job and company fit your needs.
If you have decided that you are not willing to work any overtime, then you must ask enough questions to learn if overtime will be expected, since that would mean you don't want this job.
Instead of simply stating "I won't do overtime", you might want to ask "Tell me about the work hours here." And if necessary, push a bit and say "How often are people expected to work overtime?"
From the company viewpoint, interviews are designed to see if you fit the job/role/company. Similarly, from your viewpoint, you need to see if what they are offering fits your needs.
or just do the interview normally and then refuse it if it comes up
since it's not in the contract (I know I cannot be pressured of fired
Waiting until you start work, then springing such a surprise on your employer is probably not a good idea.
While they may not be able to fire you or pressure you, I'm sure you understand that it can be an unpleasant situation all around. Within the bounds of the law and your contract, you might be denied raises, made to work shifts that you would not prefer, and given less than desirable projects. Workers who don't go along with the company culture can tend to become very unhappy.