.Net sadly is used in the most random programs and as such becomes a boon when installing programs on the network after you realise its a prerequiste that isn't included with the installer you're dealing with. Luckily it's easy to get deployed on a network! ..not..

As per norm I have supplied the MSI+MST for download.

Needed:
.Net 4.5.1 Standalone Installer
InstEd
7-Zip

Direct EXEdownload link

.Net 4.5.1 Standalone Installer (66.8MB)
http://download.microsoft.com/download/1/6/7/167F0D79-9317-48AE-AEDB-17120579F8E2/NDP451-KB2858728-x86-x64-AllOS-ENU.exe

InstEd 1.5.15.26 (4.38MB)
http://www.instedit.com/download2.html?file=InstEd-1.5.15.26.msi

7-Zip 9.20 (1.07MB)
http://downloads.sourceforge.net/project/sevenzip/7-Zip/9.20/7z920.msi

Transforms that need to be made:
CA_BlockDirectInstall = False

Steps:

First download and install 7-Zip. Then once the .Net 4.5.1 Offline installer .exe has been downloaded, right click on it, 7-Zip > Extract to 'C:\DotNet\'

With the extracted contents, run a CMD window and run this command:

msiexec /a "c:\DotNet\netfx_full_ldr_x86.msi" EXTUI=1 TARGETDIR="C:\AIP"

Make sure InstEd is installed, go to 'C:\AIP' and right click on 'NetFx_Full_LDR_x86.msi' and click 'InstEd It!'

In the menu of InstEd, click Transform > New Transform save the Transform as 'DotNet.mst' then open up the InstallExecuteSequence Table.

Edit “CA_BlockDirectInstall” to equal “False”, then in the Menu 'File' > 'Save'.

After this is done, copy all the contents of the 'AIP' folder to your deployment server's 'Dot Net' Folder. Sadly I havent been able to find a way around the MSIFileHash table to remove the localization lanuages so all the folders have to be copied...

Apply the Policy

Once that’s all done and you’ve generated/saved your Transform to the network shared "Dot Net" folder, load up Microsoft’s Group Policy Management console and right click on the Organisational Unit (OU) that you’ve added your network computers to and select “Create a GPO…”

Name the policy “Dot Net 4.5.1” or whatever you would like it to be named, then under: “Computer Configuration – Policies – Software Settings – Software Installation”

Right click in the right hand window, or on the software installation icon and choose “New… - Package”. Find the "Dot Net" folder you put on your server and select the .MSI and choose “Advanced” as the deployment method.

Once the panel opens up, go to the Modifications tab and select Add. Locate your 'DotNet.MST' Transform and click Ok. The package will now be loaded into MS GPO and you can exit out of the window as it is already saved.

Direct MSI+MST Download:

Dot NET 4.5.1 + MST (170.79MB):
http://www.mediafire.com/download/bzm8w3g2jkjn6a8/MS%20Dot%20Net%204.5.1.zip

Tips:

To find out what version of .NET is installed under Regedit check out “HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\NET Framework Setup\NDP” the versions are listed as a subkey in the 'Version' entry. The 'Version' entry is under the Client or Full subkey (under NDP).