I always find those sites are years out of date too. And things like 'cost of resealing a bath' are miles out. Easy job if you installed it and did the tiling. Hard job if you're following an installer who grouted before siliconing - takes hours and hours to clean out the joint. £57... well, if you are happy to do a bit of less skilled work at a labourer's rate, then I suppose it's about right.
Word of experience here. You will always underestimate how long things take because customers have messy houses, the last person to do plumbing in the house was slapdash, there's a lack of access to what you need to get to, and you're running a pipe past 3 electrical fittings that aren't one above the other so you're doing multiple offsets to clear them. This is okay (sort of) if all you are underestimating is the labour time and you haven't other jobs to be doing as you are 'wasting' time and 'gaining' experience. So if you want to fix prices for customers then by all means do that, but eventually you'll find yourself pricing them higher than you might if you were to work on a time basis.
I'm not the best person to ask as plumbing is a bit of a second job at present (not really out of choice... personal circumstances, you know), but, prior to all that, I had worked out what it costs me to live and based what I need to earn a working week on that. I find a working week may not be 40 hours as I spend time getting parts, doing accounts, and all the other stuff I can't actually bill people for. I charge half an hour for getting to a (local) site, then a minimum of 1 hour for labour and then in half an hour increments. Getting parts is not chargeable unless they are a specific part I cannot possibly be expected to hold in stock. Sometimes I tell people a high maximum purely as an estimate, but depends on the customer and how I know him/her. It did seem to be roughly working for me.
Bear in mind I always attribute mileage for the job against invoices and put the money in tin (literally) as I get paid. This keeps tabs on the cost of the van. Seeing this year has been low mileage and high fuel costs, the old yardstick of 45p/mile may not be enough and I may need to adjust callout costs accordingly.