Vim: inoremap works in buffer, not in .vimrc - vim

Question

I'm really eager to include the following in my .vimrc file:
inoremap <Tab> <C-x><C-u>
If I set it within a buffer (via :inoremap <Tab> <C-x><C-u>) it works exactly as I'd hoped.
But, if I place it in my .vimrc, it doesn't seem to be acknowledged at all.
I've attached my .vimrc below. A few things:
Other changes to the .vimrc file are picked up, so it's not the wrong file
Have tried adding at the very bottom of the .vimrc & still doesn't work, so it's not being overwritten
Any ideas appreciated. Many thanks.
" When started as "evim", evim.vim will already have done these settings.
if v:progname =~? "evim"
finish
endif
" Word complete
" :autocmd BufEnter * call DoWordComplete()
" let g:WC_min_len = 4
" Use Vim settings, rather than Vi settings (much better!).
" This must be first, because it changes other options as a side effect.
set nocompatible
map j gj
map k gk
set showmatch " Show matching brackets
set mat=5 " Bracket blinking
set noerrorbells " No noise
" Ruby autocomplete
" Autocomplete behaviour
set completeopt=longest,menuone
open omni completion menu closing previous if open and opening new menu without changing the text
inoremap <expr> <C-Space> (pumvisible() ? (col('.') > 1 ? '<Esc>i<Right>' : '<Esc>i') : '') .
\ '<C-x><C-o><C-r>=pumvisible() ? "\<lt>C-n>\<lt>C-p>\<lt>Down>" : ""<CR>'
" open user completion menu closing previous if open and opening new menu without changing the text
inoremap <expr> <S-Space> (pumvisible() ? (col('.') > 1 ? '<Esc>i<Right>' : '<Esc>i') : '') .
\ '<C-x><C-u><C-r>=pumvisible() ? "\<lt>C-n>\<lt>C-p>\<lt>Down>" : ""<CR>'
autocmd FileType ruby,eruby set omnifunc=rubycomplete#Complete
autocmd FileType ruby,eruby let g:rubycomplete_buffer_loading = 1
autocmd FileType ruby,eruby let g:rubycomplete_rails = 1
autocmd FileType ruby,eruby let g:rubycomplete_classes_in_global = 1
" highlight Pmenu ctermbg=238 gui=bold "improve autocomplete menu color
" Colours
highlight Pmenu ctermfg=6 ctermbg=238 guibg=grey30
" allow backspacing over everything in insert mode
set backspace=indent,eol,start
" if has("vms")
" set nobackup " do not keep a backup file, use versions instead
" else
" set backup " keep a backup file
" endif
" Set backupdir to tmp
" Do not let vim create <filename>~ backup files
set nobackup
" set nowritebackup
set history=50 " keep 50 lines of command line history
set ruler " show the cursor position all the time
set showcmd " display incomplete commands
set incsearch " do incremental searching
" Set tab to 2 spaces
set ts=2
set shiftwidth=2
" Ignore case
set ignorecase
set smartcase
" Menu autocomplete
set wildmode=longest,list
set wildmenu
" Call pathogen
call pathogen#infect()
" For Win32 GUI: remove 't' flag from 'guioptions': no tearoff menu entries
" let &guioptions = substitute(&guioptions, "t", "", "g")
" Don't use Ex mode, use Q for formatting
map Q gq
" CTRL-U in insert mode deletes a lot. Use CTRL-G u to first break undo,
" so that you can undo CTRL-U after inserting a line break.
inoremap <C-U> <C-G>u<C-U>
" In many terminal emulators the mouse works just fine, thus enable it.
if has('mouse')
set mouse=a
endif
" Switch syntax highlighting on, when the terminal has colors
" Also switch on highlighting the last used search pattern.
if &t_Co > 2 || has("gui_running")
syntax on
set hlsearch
endif
" Turn syntax highlighting on (Added by John Bayne, 18/08/2012)
syntax on
" Only do this part when compiled with support for autocommands.
if has("autocmd")
" Enable file type detection.
" Use the default filetype settings, so that mail gets 'tw' set to 72,
" 'cindent' is on in C files, etc.
" Also load indent files, to automatically do language-dependent indenting.
filetype plugin indent on
" Put these in an autocmd group, so that we can delete them easily.
augroup vimrcEx
au!
" For all text files set 'textwidth' to 78 characters.
autocmd FileType text setlocal textwidth=78
" When editing a file, always jump to the last known cursor position.
" Don't do it when the position is invalid or when inside an event handler
" (happens when dropping a file on gvim).
" Also don't do it when the mark is in the first line, that is the default
" position when opening a file.
autocmd BufReadPost *
\ if line("'\"") > 1 && line("'\"") <= line("$") |
\ exe "normal! g`\"" |
\ endif
augroup END
else
set autoindent " always set autoindenting on
endif " has("autocmd")
" Convenient command to see the difference between the current buffer and the
" file it was loaded from, thus the changes you made.
" Only define it when not defined already.
if !exists(":DiffOrig")
command DiffOrig vert new | set bt=nofile | r ++edit # | 0d_ | diffthis
\ | wincmd p | diffthis
endif
" My customisations
" No backup files
set nobackup
set nowritebackup
set noswapfile
" Whitespace identifier
:highlight ExtraWhitespace ctermbg=darkgreen guibg=lightgreen
:match ExtraWhitespace /\s\+$/
" Colour options
" color codeschool
" set guifont=Monaco:h12
" let g:NERDTreeWinPos = "right"
" set guioptions-=T " Removes top toolbar
" set guioptions-=r " Removes right hand scroll bar
" set go-=L " Removes left hand scroll bar
" autocmd User Rails let b:surround_{char2nr('-')} = "<% \r %>" " displays <% %> correctly
" :set cpoptions+=$ " puts a $ marker for the end of words/lines in cw/c$ commands
" Tab behaviour for ultisnips
let g:UltiSnipsExpandTrigger="<tab>"
let g:UltiSnipsJumpForwardTrigger="<tab>"
let g:UltiSnipsJumpBackwardTrigger="<s-tab>"
inoremap <Tab> <C-x><C-u>

Answer 1

In this case, :scriptnames, while useful, is not the best way to see where a mapping was last set.
:verbose map <tab>
is a lot more precise.

Answer 2

Maybe another script is overwriting it after your .vimrc is read.
You can use :scriptnames to see what files are read on startup.