Don't know about "Z" training, but in my time I automatically went onto the long term reserves & could theoretically be called back up, up to my 46th. This came about due to the Army act of 1996, prior to that I was liable to reserve service upto the 22 year point (taking into account any previous Colour service).
You were expected once a year at a designated time & place to present yourself, with full reservist kit (stuff you had to keep hold of) & partake in a set of T.O.E.T.S. (test of elementary training skills) a physical all to a particular standard & for this you were paid a couple of bob for your time & efforts.
This was later commuted (to save money) to filling out a reporting form which you were sent once a year to let them know you were still breathing & available as cannon fodder. All this so that should Her Maj's Government decide a bout of putting Johnny foreigner in his place was overdue, sufficient meat for the mincer was available.
I believe this practice is nowadays redundant what with, 'Uman rights & suchlike.
Only mention of "Z" I could see was :-
Reserves and reservists the British Army of 1914-1918