I would not bother with 'accelerator' as you will get same or better by enduring the bellwire on terminal 3 on your master and extensions is disconencted as it can act like anaerial for noise
there would be no charge for an engineer visit if problem is not in your home the charge is advisory if there is a problem in your home with wiring or damage to socket. as you are sure problem not in your home then there should be no charge. there is no charge if no fault found
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