I'll soon be starting a small 2 story extension, can someone please advise me if I can mix my own concrete for the foundation strip or do I have to buy pre-mix? If I can mix myself what ratio of sand / cement should be used.
Firstly, don't buy sand for your concrete because you will not get concrete but mortar instead. You can mix your own but you will need to buy all-in ballast and ordinary Portland cement.
If your extension is of a small size then mix your own, but bare in mind that a small building does not always equate to small foundation's. If you do mix your own, you need to try and pour the whole foundation in one go. If that's not possible you must join each section with re-bar to prevent differential settlement.
And finally, if you can afford it, get ready mixed delivered and pumped if access is a problem. Knocking up concrete with a mixer is hard work and having it produced in a plant will ensure a consistent mix to work with, and a guaranteed structural load bearing capacity.