Answered what I can. Many here much more expert than me!
1 question I have is, where do I find my Apple I.D. or account?
Easiest is to go to
https://appleid.apple.com/ Make sure you remember your Apple ID - log is somewhere safe as it's a pain if you forget.
https://support.apple.com/en-us/HT203993
Apparently, I bought Cloud through Curry's, so I probably don't need iCloud?
You get 5GB of free storage with iCloud; more with a monthly subscription. If Currys have sold you theirs, it's not economic to boost the Apple version but see how much you use over the next 12 months and compare with prices with Apple. Some people (like me) don't use the cloud very much. Except for calendar, contacts, etc which iCloud syncs to my other Apple devices, I rely on my external hard drive for back up.
I bought a 2TB external hard drive, which I haven't installed yet; I'm assuming that needs to be formatted? http://www.macworld.co.uk/how-to/mac/how-format-storage-drive-for-mac-usb-3541758/
Where I found the spec. details of my Mac, I saw a button saying Software Update - I don't need this?
Clicking on this will tell you whether or not an update is available. You could have the latest OS (High Sierra) or Sierra 10.12.6 installed. If 'High Sierra' is not already installed, you may like to wait until teething problems are sorted before updating. I am. Depending on what you have on your Mac, others may suggest extra 'insurance'.
When I've installed the hard drive, do I back-up my Mac onto it and how do I do this please?
Plug the drive into a USB and go to the toolbar and find Time Machine - the clock symbol. Click on 'Open Time Machine Preferences' > Select Drive and select your hard drive. The first back up will take a-g-e-s but subsequent ones are usually quick because only the new data is copied. When it's finished you need to open Finder (square blue/white square with smiley face in Dock) and eject the hard drive by clicking on the arrow beside it, or dragging the name to Trash (waste bin icon in Dock). Any peripherals like this always have to be dismounted otherwise you get messages saying there may be damage.
Don't be tempted to install any virus protection software - and I hope Currys haven't talked you into buying any (if so, don't install and ask for a refund) - as Macs do not need extra protection. Something for malware (Malware Bytes, free) is advised. Onyx (free) is recommended for identifying and cleaning up.
GOOD LUCK