Fully tested and working 100%

The IHS subroutine now looks like this...
G91 (switch to incremental mode)
G01 X28.00 Y4.00 F6000 (position probe switch)
G90 (switch to absolute mode)
M1050 (trigger the probe cylinder)
F1300
G28.1 Z5.000 (probe the surface)
G92 Z#15045 (apply the probe switch offset from settings screen DRO)
M1051 (raise the probe)
G04 P0.2
G91 (switch to incremental mode)
G01 X-28.00 Y-4.00 F6000 (put the axes back to where we came from)
G90 (switch to absolute mode)
M99
%
You may notice that the first G04 P.0 has been removed - since changing to macro's rather than the brain the odd screech disappeared and it does not need the extra P04, I have no idea why but it works. The second P04 is needed as it allows time for the probe to lift clear of the plate before it moves.
Are any of the Abs/Inc switches surplus???