Hi John, Thank you for the reply. I think with your questions and (quite) a bit trial and error I may have resolved the situation, although I still try to understand what was happening… In any case, I think the $10=3 was wrong and had to be $10=2. I have had problems before with this as I tried to get the z-axis to work properly in LB, so that may be the root of the problem, although I can’t understand why it used to go right and suddenly it went pear shaped. Gremlins may be? 
With regard to your questions:
The 3018 ProVer (Sainsmart) is default delivered with limit switches, so the GRBL build may have been updated before it was shipped.
In the 3018 to work properly in LB I need to set the X and Y (and Z) co-ordinates positive. This happens with the macro:
$10=2 (previously $10=3)
$30=100
$32=1
$110=1000
$111=1000
$120=3000
$121=3000
$122=3000
G10 L2 P1 X-253 Y-316
$H
The 3018 keeps this data, while the S9 doesn’t keep this data. So for the S9, everytime I start up I send an initialization string which works fine for me:
$10=0
$22=1
$23=3
$24=100
$25=2000
$26=50
$110=6000
$111=6000
$120=1000
$121=1000
$130=390
$131=400
G10 L2 P1 X-387 Y-397
$H
This is the settings for the 3018:
$$
$0=10
$1=25
$2=0
$3=2
$4=0
$5=0
$6=0
$10=2
$11=0.010
$12=0.002
$13=0
$20=0
$21=1
$22=1
$23=3
$24=25.000
$25=500.000
$26=250
$27=1.000
$30=100
$31=0
$32=1
$100=794.000
$101=800.000
$102=800.000
$110=1000.000
$111=1000.000
$112=600.000
$120=3000.000
$121=3000.000
$122=3000.000
$130=263.000
$131=316.000
$132=40.000
ok
And this for the Sculpfun:
$$
$0=10
$1=25
$2=0
$3=0
$4=0
$5=0
$6=0
$10=0
$11=0.010
$12=0.002
$13=0
$20=0
$21=0
$22=1
$23=3
$24=100.000
$25=2000.000
$26=50
$27=3.000
$30=100
$31=0
$32=1
$100=80.000
$101=80.000
$102=250.000
$110=6000.000
$111=6000.000
$112=1000.000
$120=1000.000
$121=1000.000
$122=3000.000
$130=390.000
$131=400.000
$132=200.000
ok
So, as said, at the moment it seems to work as expected. I will do some more testing and studying later on - as it still puzzles me why the right error works ok, and left goes haywire. But I appreciate you help as it triggered the resolution. Thank you! 