The problem with many of these RTR models is the receiver. If it does not have a crystal in it then it is a wide band receiver and any other transmitter in the band will stop it working properly. If you cannot get to the receiver then check the crystal in the transmitter. If it is marked 40.680MHz then it is likely to have a wide band receiver. Takes me back to the 1950 era when we all had to use this type of radio and only one model could be run at a time.
Your other question on 49MHz. Yes, it is a legal band for toys in the UK. If you can get a truck on this band then you should not have a problem running with your 40MHz ferry.
Fred