how to 'fix' a half finished print..

[alaris2 1]

so 5.30 through my 6hr print the z stage hits a piece of scrap plastic I carelessly left at the bottom and jams...

I've seen a couple of people try to rescue prints, usually their print froze, so they knew where they were. but this kept printing several layers (which are of course all a mess)

I can count the number of erroneous layers - so I need to beg for a crash course in g-code now. is there some way I can ask the printer where it currently is in z?

I can then subtract n layers, find the relevant G1 ... Z in the gcode and snip everything before that point right?

are there any parts in the start gcode that I need to keep? I don't really want it to try and home the x,y,z obviously! and I think I need to reset the extruder so it doesn't try to extrude a stupid amount of filament - I forget the g-code for this :( but I think I need to keep the first 2 lines (M92.. and M109.. ) right?

thanks to anyone who helps!

my start g-code looks like -

;TYPE:CUSTOM

M92 E1075.04754031

M109 S210.0

G21 ;metric values

G90 ;absolute positioning

G28 X0 Y0 ;move X/Y to min endstops

G28 Z0 ;move Z to min endstops

G92 X0 Y0 Z0 E0 ;reset software position to front/left/z=0.0

G1 Z15.0 F400

G92 E0 ;zero the extruded length

G1 F75 E5

G1 E3.5

G92 E0 ;zero the extruded length again

G1 X100.0 Y100.0 F3500

G1 Z0.0 F400

G90 ;set positioning to absolute

G21 ;set units to millimeters

M113 S1.0

M108 S25.0

;LAYER:0

M106 S255

;TYPE:SKIRT