That's really a question to pose to your service provider.
Openreach's charges are publically available here:
No substantial changes since 2009.
In a repair scenario Openreach (OR) levy charges onto the service provider (SP) if the fault is proved not to be on their network i.e. not on the copper line and associated exchange jumpering. So a fault caused by something in the end user premises or by the SP's equipment in the exchange would be chargeable. If the SP elects to arrange an OR engineer visit to their end user's premises then it can get expensive if the fault is not an OR responsibility fault.
Choosing to send an OR engineer is the SP's choice.
SP's pass on these charges to customers as they see fit.
As stated elsewhere in this thread, if OR find a fault on their infrastructure then no charge is levied on the SP, unless the SP has requested an engineer visit outside the constraints of the agreed service contract (e.g. expedited out of hours visit for a standard care circuit).
The charging structure is quite complicated depending on what product and service level the SP is paying OR for. I'm not aware of any scenario where OR requires an upfront payment.
Care to elaborate on who your supplier is, what product (voice / dsl / something else) you have the fault with and the nature of the fault?