set n=1
:starline
for %%j in (*.txt) do (
:3
if exist D:\work\test\new_%n%.txt (set /a n+=1&goto 3)
set file=%n%.txt
for /f "skip=1 delims=" %%i in ('type "%file%"') do (
echo %%i >>D:\work\test\new_%n%.txt
)
set /a n+=1&goto starline
)
::start "" D:\work\test\new_%n%.txt
::pause>nul

@echo off
for /f "delims=" %%a in ('dir/b *.txt') do (
type "%%a">>All.txt
)