C Hanlon An All Trades Company

Electrician to fix shower. The charge for first half hour is double that for subsequent half hour intervals. Problem in switch was determined fairly quickly. The part wasn't carried so was informed it would take 10 minutes to pick one up. Given the charge structure above and the fact that the company address is a stones throw from where I live, I agreed. On leaving, the electrician mused on where he would go to pick up the part. An hour later, he returned. The shower is working again.
So the question would be how the pick up time could be stated at 10 minutes when the location to find a switch had not yet been determined and why would this part not be held by an electrician(or at the company address) going to check a shower, given that this is likely to be at fault in a large number of shower faults ?