If you choose to accomplish repairs on your Sym HD 125 then it is always a great idea to have a Workshop Manual to use as reference if perhaps you need to know the best way to take off a specific part for example Gearbox and match it again accurately.
Sym HD 125 Workshop Manuals will give you exact diagnostic info for your vehicle so if something is not working it will help you find out what the problem is and ways to resolve it. You can then assess if this can be achieved on your own or that the job is too big you could make a booking for your HD 125 right into a nearby mechanic.
Sym HD 125 manual