If you need to perform maintenance tasks on your own Tomos BT then it is always a good idea to own a Workshop Manual to use as a reference in case you need to know the best way to take off a particular part much like the Gearbox and match it again efficiently.
Tomos BT Workshop Manuals will give certain diagnostic information with regards to your car so if something is broken it will help you find out what the problem is and how to resolve it. You can then determine if this can be achieved on your own or in the event the job is too big you can actually make a reservation for your BT into a trusted friendly mechanic.
Tomos BT manual