Yay! Just changed my coolant expansion tank on my 2014 25th Anniversary Edition NC/Mk3. It’s a 10 minute job (bit longer if it decides to rain again). The prevous owner had the coolant changed in March BUT not the tank!
I found it helpful to slacken off the bonnet strut nut so as to give more wriggle room for removing the thick pipe on the underside of the tank.
Make sure that you siphon out the tank before you remove it and that you have something to collect the coolant that inevitably tries to escape.
The recomended interval for changing the FL22 coolant is 10 years.
Just to be pedantic, on the NC it’s not really an “expansion tank” but a pressurised part of the cooling system. Also, I think that the recommended interval for FL22 is actually 11 years, not 10 
Not sure that the tanks really need replacing, unless you’re starting to see fractures at the neck (I check mine whenever I do a fluid check). Most of the failures seem to have been in the hotter states of the USA. There have been very, very few UK failures reported on the various forums, although failure can be catastrophic to the engine, as you get little or no warning of overheating.
