The charge for an internal fault is surely common knowledge?
From a legal perspective, what matters are the terms of the contract between the service provider and the consumer.
There are certain terms implied into a contract by statute - the fairly well known consumer rights created by the Sale of Goods Act 1979 and Supply of Goods and Services Act 1982 fall into this category.
Terms can be implied in certain other circumstances, but these are relatively narrow. There are four scenarios that come to mind:
- when the term is customary in a certain locality or trade
- when the term is a consistent part of dealings between the parties
- when the meaning of a contract is unclear to an observer, but the parties were in agreement over what was intended at the time of contract formation
- when the contract is silent over necessary customary terms
Scenario 1 is concerned with common knowledge, but really only covers matters that are so well known within the locality or trade in question that there should be little doubt in the minds of both parties that the term is incorporated even in the absence of an explicit term covering the matter.
It is hard to see how scenario 2 could apply to a residential ISP, as few consumers will take out enough contracts in a year to be regarded as consistent dealings.
Scenario 3 doesn't apply here, as it would be impossible to show the customer agreed to visit charges in 'no fault found' scenarios if the contract was silent on the matter. The case law here is clear that this scenario is only about resolving disputes about the meaning of the contract as it stands, and it is not possible to improve upon the contract (so the ISP couldn't use this scenario to argue 'we left out mention of visit charges, but the customer should have known they would be levied in some scenarios').
Scenario 4 doesn't apply here, as there is nothing necessary about passing on a visit charge. The case law here relates to a landlord arguing they had no responsibility to make reasonable efforts to maintain the stairs, lifts and rubbish chutes in a block of flats. The court held that an easement to use the stairs, lifts and rubbish chutes was necessary for the tenant to use their flat, so the landlord had a contractual repair obligation over these necessary areas.
I would expect that the only legally enforceable way a residential ISP could pass on a visit charge is via an explicit contractual term - either in the contract to provide service, or a contract relating to the visit (for example, if the customer agreed to the visit on the basis that a charge would be levied if no fault is found). A contract can be formed verbally, though the party attempting to rely on that contract would have to be able to demonstrate a contract more likely than not existed, for example by producing a recording of a telephone call.
To my mind, an ISP saying on the phone "We can send an engineer, but if the engineer reports no fault is found, or the only faults are on your internal wiring, we will charge you £150. Do you agree?" and the customer responding "Yes" forms a valid contract that allows the visit charge to be passed on.