This may sound stupid but am I understanding this correctly in that the mining of bitcoins is actually people doing work on the growing of the block chains' and are rewarded BTC for an amount of work performed.
Essentialy, yes. Miners are securing the system by providing computational power, they're making it harder for all others to mess around with the block chain.
If that is correct who is the agency responsible for the reward?
There's no agency. It's all in the software and you can look at the code here, it's open source:
https://github.com/bitcoin/bitcoinNoob question 2: If you buy 100BTC or 2BTC or 1 BTC would in any of these scenarios you only obtain one private key, or would you receive 100 keys in the first case.
A private key is like the password for your account (or your bitcoin "address"). Whether you send 100 bitcoins or just 1 bitcoin to your address, the private key for this address remains the same. So when you buy bitcoins at an exchange you most likely will send them to one of your addresses and there's only one private key per bitcoin address.