I always go for same day fix, if I can. I find most of the bigger guys hold really poor choice of stock and you need to order things like primary thermistor for a vokera and get it next day. I use an independent who pretty mush never let me down. They are a little more expensive for most parts, but they have them because their finger is on the pulse for what spares are selling in my district. I never use internet. If independent does not have it I order it through them for next day, usually. Most customers want stuff china cheap so add cost to parts and show them Plumbcentre's website that shows the RRP price. They don't like to think that you have the audacity to actually earn a living from your labour charge. If I make a mistake on diagnostic then I either take the part in to my stock or depending on part come to an arrangement that they pay for all parts and installation of the correct part and my time is free for the mistake.