I followed a few posts of yours roxy, excelletn work by you and oldmcg. my process was

M502
M500
g28
g29 p1
g29 T
g29 s1
g29 p3 t
g29 p3 t
g29 s1
g29 a
m500
reset the machine
then tried a g29w says its active.
then g26 which made it drag across the bed as if its too low

I did wonder wether it was my z-probe offset that was wrong but i tried everythign from -1.4 to 4.5 with the same result. My probe is on my nozzle it activates about .4 lower than the bed (if that makes sense)