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But don't have a bench, what can I mount it to?
black and decker work mate
You can get clamp vise's that you can clamp to any surface really, up to you what you prefer, and of course you can protect with some offcuts of wood or whatever, this way you could maybe use it on a kitchen work surface or normal table, or even a wooden chair or whatever you have around if only using the clamp for short periods.
If it's not for bigly jobs get something like this
https://www.screwfix.com/p/magnusson-portable-vice-swivel-base-3/5324v
One of these bolted into the ground:

Works just as well for a Vise as it does for a Grinder (within reason, and so long as its bolted down).
As above its what I had set up. Better as theres no workbench in the way
black and decker work mate
Previous owner of this house left one in the shed. It is a proper good one, all cast aluminium and bushings on the mechanism.
It has a surprisingly strong vice action / clamping on it - could you get away without a proper vice? Or, as said above, mount one on a folding workstand.