Space/Delete/Caps keys only work with Fn pressed. Apple wireless keyboard.

A few weeks ago i noticed that certain keys weren't working on my keyboard.

A row of central keys, delete, function, caps, arrow keys and the space bar. De-coupling the keyboard to attach to my iPad rendered the same results. No use of keys. I gave it a clean but zilch. So I bought a replacement apple wireless keyboard from a friend, not new but un-used.

It worked PERFECTLY.

BUT after the weekend, I sit at my desk to discover that while the central row of keys works fine, I have no delete key, no alt key, no down-arrow key, no spacebar, no caps...the capslock light doesn't even turn on.

I start going crazy. Since then i've:

I've cleaned the keyboard,

I've deleted relevant library files,

I've reset default keyboard function keys,

I've performed SMC/PRAM resets.

Nothing.

My keyboard - now two of my keyboards (and I only have two) are unusable.

But today I discover that in fact all the keys DO work. But ONLY if I press and hold the fn key in the bottom left hand corner. If I press the fn key the capslock button works, the light even turns on. if I press it down, I can delete things, use the alt key, I can make spaces between texts, I can use my down-arrow.

As you can imagine, this took a while to write.

I'm going out of my mind. PLEASE GOD SOMEONE HELP ME...