History:
8 years of domestic and industrial work.
Answers:
Sounds like it could be to do with the oven being connected to a smaller sized circuit breaker than previously, overloading the circuit which would then trip the breaker.
Would need to run a cable from your consumer unit in your house rather than garage, the amount would really depend on what needs done, is hard to have a rough idea until it is looked at.
If the bathroom is wired to the original consumer unit with no rcd protection then it would need to be rcd protected.