20 minutes' labour is 20 minutes' labour if you bring the your house to me and I can do the work at my leisure 🙂 If I have to come out to a customer, though, a 20 minute job can be my morning gone. I understand the customer's point of view, but ultimately I charge what I need to charge to avoid finding going to work at a minimum wage job on contractual hours an attractive proposition.
To give you a more helpful answer, in South-East (UK) prices, if it were a local job (<5 ,miles each way), I would estimate I'd be charging you £60 labour for that 20 minute job (assuming it's no more than an hour from the time I set foot on your property to the time I'm packed up and have started driving away) plus (the cost I've paid for the part+10%). I don't have good buying terms with my merchants, so probably you can get the part as cheaply as I could supply it. That is if I hadn't agreed a price before I started work, because if you wanted a fixed price, I'd be adding a duck-up factor for when the 20 minute job ends up taking me two hours.
Bear in mind, a tempering valve (assuming you mean a thermostatic mixing valve on the hot water coming to your taps) is an expensive part I would not have sitting in the van just in case I needed it, so unless I knew exactly what I needed before turning up, I'd be driving around town to buy it specially and the travel time would be chargeable. Hope this helps.