To upgrade from OS X 10.5.8, you must first buy Snow Leopard by calling the phone number in the Apple Online Store: 800-MY-APPLE (in the US)
Make sure your system meets Snow Leopard's requirements:
To install Snow Leopard for the first time, you must have a Mac with:
- An Intel processor
- An internal or external DVD drive, or DVD or CD Sharing
- At least 1 GB of RAM (additional RAM is recommended)
- A built-in display or a display connected to an Apple-supplied video card supported by your computer
- At least 5 GB of disk space available, or 7 GB of disk space if you install the developer tools
After you install Snow Leopard you can decide if you want to upgrade OS X to the latest version, Mountain Lion. Mountain Lion is available in the Mac App Store, which is available to you after upgrading to Snow Leopard.
Make sure your system meets the requirements for Mountain Lion:
Mac OS X 10.8 "Mountain Lion" System Requirements
To install Mountain Lion, you need one of these Macs:
- iMac (Mid 2007 or newer)
- MacBook (Late 2008 Aluminum, or Early 2009 or newer)
- MacBook Pro (Mid/Late 2007 or newer)
- MacBook Air (Late 2008 or newer)
- Mac mini (Early 2009 or newer)
- Mac Pro (Early 2008 or newer)
- Xserve (Early 2009)
Your Mac needs:
- OS X v10.6.8 or OS X Lion already installed
- 2 GB or more of memory (RAM)
- 8 GB or more of available hard disk space
If your system does not support Mountain Lion's requirements, you may be able to upgrade to Lion. To do that you must call Apple again to order a download code. Redeem the download code in the App Store to download Lion. Lion costs 99¢ more than Snow Leopard.
Make sure your system meets the requirements for Lion:
Mac OS X 10.7 "Lion" System Requirements
- Mac with an Intel Core 2 Duo, Core i3, Core i5, Core i7, or Xeon CPU
Core Duo CPUs are not sufficient for Lion
- 2GB of memory (RAM)
- OS X v10.6.6 or later (v10.6.8 recommended)
To determine which Mac you have, what kind of CPU it uses, and how much memory is installed: > About this Mac
The specified 2 GB minimum is barely adequate for either Lion or ML. They will run in 2 GB but you may not like its performance. 4 GB is a more realistic minimum.
At the very bottom of this page, do you see something similar to this?
I'm not in the UK so I don't know how your page appears. If you click the link Apple Online Store on the US site, the Apple Sales phone number appears at the upper left.
See if that works for you, in the meantime I will try to find the equivalent number in the UK.