I disassembled my left hand GMA action to bead blast the cocking piece (was very shiny). After having re-assembled the bolt I cannot get the bolt to slide in. It looks like this and won’t push forward over the magazine :
https://imgur.com/a/I9Vm7
What have I done and how do I rectify the problem? Many thanks.

Your striker is in the fired position, it cannot be fully screwed back into the bolt body in that position.

You need to retract the striker and engage the safety to the mid position, then you should be able to screw the firing mechanism back into full battery.

Remove the firing pin/cocking piece assembly from the bolt body, place the tip of the firing pin on a piece of hardwood (like a cutting board) and press straight downward on the cocking piece compressing the main spring until you can engage the safety to the mid position. You should then be able to screw it into the proper position.

And make sure that you are pressing absolutely straight down, if you are off at an angle you could bend or snap off the firing pin tip.

I am not sure that I am explaining this very clearly, maybe the best way for you to understand is to check out a You Tube video on disassembling a M98 firing mechanism.

Nice rifle, BTW!

Thank you Sir, that's what I thought would do the trick but the bolt sleeve is stuck so that it won't unscrew.

OK...

Then you will need to retract the firing pin by pushing rearward on the bottom of the cocking piece. Remove the bolt from the rifle first.

Then you need to place the bottom of the cocking piece against the edge of a wooden workbench (or something similar) and pull down on the bolt body, retracting the firing pin. Then with the FP retracted you will need to rotate the bolt body until the safety can be pulled back to the mid-position. Then screw the firing mechanism fully into position and you should be good to go.

It is easier to do than to explain, if anyone can describe it better, feel free.

Thank you Sir, it worked like a charm!