I can't help you with GRBL and my comment isn't going to solve anything for you, but that machine looks like a toy. It is not made for anything where power is needed, it contains a lot of plastic stuff (looks like cheap 3-D printed things) where metal would be preferable but not less than at least precision machined Delrin. The steppers seems too small, the screws have no bearing supports on the motor end, are made for 3-D printer, uses unsupported curtain rods and so on... so of course it is weak. All that can be seen in the eBay link pictures already. I am afraid that you threw £180 out of the window.