@OP – DIY. In the time it takes to package it and trot down to the postt office, you’ve done the job. I changed the pistons and seals on my Mono Minis it was a doddle. When I did my M4s I changed the pistons because one was scored, I didn’t need to change the seals at all.
The only specific tool you need is the piston cover remval tools, a few quid from CRC then you got it forever. If you are near Dorking you can come and borrow mine.
Before you start, read through the page I linked, visualise each step, break it down. That page has a lot of detail, the job is easier than it looks once you get it straight in your head. You will need a well lit place to work with some clean space for the bits.
It’s only a casting, some pistons and some rubber seals, it’s not rocket science, it’s not a dark art. It’s a good thing to know how your bike parts work, you’ll have confidence to do more in the future, and you’ll diagnose problems qucker and more accurately.
Servicing my M4s (which I did because neither of my proper-mtb LBSs fixed them properly when they were grabbing and I got fed up with taking the bike back and forth to them) set me down the path of building my own bike. It’s a whole new world!