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I downloaded AutoDMG a while ago, never really understood how to use it? Didn't find a program to actually run? So wasn't really sure what to do! Do I have to create a full image of the laptop? With all our binding and base done? As we don't mind running our current 10.10.5 image on it, it doesn't have to run 10.11, in fact we would prefer to run 10.10.5 as this would be the only 10.11 machine and we have one app I;ve been told that doesn't fully work on 10.11?

Would this change anything!

AutoDMG is downloaded as a DMG, with the AutoDMG app inside

To use:

Download the OS X installer from the App Store
Drag the installer from /Applications to the AutoDMG to the area indicated
The app will then check to see if there are any software updates that can be applied.
Hit Build and the app will convert the installer to a DMG file.

You can find more information on the wiki: https://github.com/MagerValp/AutoDMG/wiki/Getting-Started

You can then use this DMG for modular/thin imaging (which saves a whole load of time as you just swap out the DMG everytime there is an update, rather than needing to build a monolithic image) or as a base for Netboot images.

Is there no way of just creating a new NBI so we can still image the machine with our 10.10.5 build? As OneDrive does't fully work with 10.11 and it's something we use across the business.

So just the NBI without a whole new image and configuration?

DOUBLE POST.

If the Mac shipped with 10.11 and won't boot to 10.10 then you'll have to deploy 10.11 to it unfortunately.

@betty02 you don't need to update your NetBoot Server's OS to 10.11 in order to have a 10.11 NBI that you use for imaging. But as @davidacland stated, if the computer shipped with 10.11 then you would need to create a 10.11 NBI that you will have to use with at least that new computer. You can have more than one NBI on a NetBoot server. Our Mac OS X server that we use for NetBooting is currently on Mac OS 10.10.5 and I have 2 different NBI's on it for imaging purposes. @andysemak gave you a good place to start with creating the Base OS using AutoDMG. There are some other good threads on JAMF Nation on AutoCasperNBI or you can look at Ben Tom's MacMule site for more detail. Both AutoDMG and AutoCasperNBI are very user friendly tools so once you've take a look at the getting started information for them you really shouldn't have any issues using them.

Chaps! Thanks for the help!

So I used AutoDMG and created a DMG off a fresh install, then used the CasperNBI software, added it to the share and booted to the Mac 10.11 NetBoot that was listed, chose my 10.10.5 Base Installer and it's currently block copying so fingers crossed!!!

All looks good guys! Was actually quite an easy task to be fair!! Roll on 10.12!! Haha!
Thanks again!

@betty02 if you are putting 10.10.5 down as the OS for the iMac it may not boot properly if the iMac shipped with 10.11. Some computers require the OS they are shipped with to boot properly. If the computer was released by Apple after Mac OS 10.11 first shipped then you are likely going to have to use 10.11 on the computer. Otherwise you may experience weird behavior with the computer using an older OS than what it was shipped with.

@mpermann Given it a few restarts and everything looks to be working OK? It was shipped with 10.11 but appears to be working OK? Anything in particular I can check? Its the same model number as some of our other ones and image copied and booted OK so far?

Just because a Mac ships with the latest OS doesn't mean the hardware requires that OS to boot. Its Apple's SOP to start updating shipping Macs with their latest OS as soon as it goes public, but unless the hardware has been revved between the time the new and previous build of the OS was released, it may boot fine off the older version.
A good source to check on this is Mactracker. It lists the Original OS under the Software tab, which would be the oldest OS version the Mac should be able to boot from.
All that being said, I have seen some exceptions in the past where a Mac that has had a firmware update applied will then only boot from the latest OS, and can't be rolled back, even if that model originally shipped from Apple with an older version.

@betty02 if the computer is the same model as other computers that originally shipped with 10.10 then you'll be fine. What you have to be careful of is if it's a brand new model that was released after 10.11 was released. In those instances, you want to make sure you use at least the shipping OS or newer. Sometimes Apple will fork an OS release on a new model machine and you must use the shipping OS because it's the newest version available. There are some other threads on here that talk about forked OSes that would give some more detail on the issue.

Edit: what @mm2270 said. :-)