I would like to downgrade from Windows 8 Pro to Windows 7 but I only have the Windows 7 Upgrade disk (Home Premium) - will this work? If so can I just whack in the disk or are there specific instructions. Any help much appreciated.

Clean Install Windows 7 with Upgrade Media
http://www.winsupersite.com/article/windows-7/clean-install-windows-7-with-upgrade-media-128512

Clean Install Windows 7 With Upgrade Media and Product Key on Formatted or Empty Blank Hard Drive
http://www.mydigitallife.info/2009/10/27/clean-install-windows-7-with-upgrade-media-and-product-key-on-formatted-or-empty-blank-hard-drive/

Please read the following:
The upgrade will check that (Windows XP or Vista) is installed. However the Windows 7 End User License Agreement does specifically state that, therefore as long as you have a valid COA and product key for the computer you wish to upgrade you can.

So if your computer came with a previous qualifying license (Vista or Windows XP). Note: The Certificate of Authencity (COA) must be attached to the computer and readable, then the COA is reason enough for representing compliance to the software license agreement. No actual installation of previous software necessary, which means you can do a clean install with Windows 7 Upgrade disk

-------- License ---------------

"15. UPGRADES. To use upgrade software, you must first be licensed for the software that is eligible for the upgrade. Upon upgrade, this agreement takes the place of the agreement for the software you upgraded from. After you upgrade, you may no longer use the software you upgraded from."

16.PROOF OF LICENSE.
a.Genuine Proof of License. If you acquired the software on a disc or other media, your proof of license is the genuine Microsoft certificate of authenticity label with the accompanying genuine product key, and your proof of purchase. If you purchased and downloaded the software online, your proof of license is the genuine Microsoft product key for the software which you received with your purchase, and your proof of purchase from an authorized electronic supplier of genuine Microsoft software. Proof of purchase may be subject to verification by your merchant’s records.

--------- Phone Activation is required -------------------

How to activate Windows 7 or Vista manually (activate by phone)
1) Click Start, and in the Search box type: slui.exe 4
2) Next press the 'ENTER' key
3) Select your 'Country' from the list.
4) Choose the 'Phone Activation' option.
5) Stay on the phone (do not select/press any options) and wait
for a person to help you with activation.
6) Explain your problem clearly to the support person.
7) The support person should give you a confirmation ID, copy it down on paper,
8) Verify the ID is correct by reading back to the support person.
9) Enter the ID number and click 'Next' to finish the activation process.

-------------- Alternatives -------------------

Microsoft Activation Centers Worldwide Telephone Numbers: http://www.microsoft.com/licensing/existing-customers/activation-centers.aspx
(This site is for Volume License Activation but if you call they will help you)