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But are they a rip off?
You were lucky enough to be in a position where you could get the part and repair done for £20.
Was it done as a favour? A mate of yours doing the job without overheads, not charging tax & VAT?
Not everyone can be that lucky, they have to rely on a prompt & professional service to get them mobile again.
I hope this minor problem of a puncture is the worst problem you have with your mobility scooter, however, what if something more serious or technical was to go wrong with it? Would your mate be able to provide the parts and know how needed to get you mobile or would you turn to a company that has been there for years serving the needs of folk such as you?
Perhaps you judge them too harshly?