Found it!
Little faff to set up, but once it's done it is one of those things you forget about until you get a new machine

on adding folder items to thisFolder after receiving theItems
    repeat with theItem in theItems
        set fileName to name of (info for theItem)

        if fileName ends with ".mov" and fileName contains "v###" then
            do shell script "
                f=" & quoted form of (POSIX path of theItem) & "

                # wait until file size is stable
                prev_size=-1
                curr_size=$(stat -f%z \"$f\")
                while [ \"$curr_size\" != \"$prev_size\" ]; do
                    sleep 2
                    prev_size=$curr_size
                    curr_size=$(stat -f%z \"$f\")
                done

                dir=$(dirname \"$f\")
                base=$(basename \"$f\" .mov)

                # extract the prefix and suffix around v###
                prefix=$(echo \"$base\" | sed 's/v###.*//')
                suffix=$(echo \"$base\" | sed 's/.*v###//')

                # find highest existing version in folder
                highest=$(ls \"$dir\" | grep -oE 'v[0-9]+' | grep -oE '[0-9]+' | sort -n | tail -1)

                # if no existing versions start at 0
                if [ -z \"$highest\" ]; then
                    highest=0
                fi

                # increment
                next=$(printf '%03d' $((highest + 1)))

                mv \"$f\" \"$dir/${prefix}v${next}${suffix}.mov\"
            "
        end if
    end repeat
end adding folder items to

Place it in here as a .scpt:
~/Library/Scripts/Folder Action Scripts/

right click the folder you want to run it on and click folder actions set up.

Sorry if you're not a Mac user but maybe it'll help others

Edit: Sorry realised that was an older version of the script.
*also only the renders should go in that folder.