I've tried to do this job and I am reasonably good at DIY and repairs.
Whilst it's easy to see what has got to be done retensioning the system can be tricky and if you don't get wire correctly positioned on the rollers it can be painful.
Having not got it quite right when I did this, the garage door closed on my hand with the new cable wrapped round my fingers. At the time I said out loud "this is going to hurt" and I was right.
Most jobs round the house I deal with, however paying £50.00 for someone who knew what they were doing was good value and I would highly recommend this.
You can't ride very well with a handfull of fingers that look like jumbo sausages.