Thank you for your replies...
So when the actual main wallet is encrypted with a passphrase this encryption and same passphrase is automatically applied to the wallet.dat back up that the user then makes.
As long as the backup occurs AFTER the encryption, yes.
Only one encryption needs to be done...because the wallet.dat file ...*is*...the wallet.
Yes.
Back ups need to be carried out preferably before 100 transactions have been completed...is this regardless of whether or not I change my address to receive coins though..?
With the default settings, the wallet pre-generates the next 100 addresses that it will use. Those addresses are hidden from you, but are all stored in the wallet.dat (and already encrypted if you encrypted before backing up the wallet.dat).
Your wallet retrieves one of these 100 addresses every time you tell the wallet that you want a new receiving address.
Your wallet also uses one of these 100 addresses every time you
send any transaction from the wallet (the address is kept hidden from you, but used to send change from the transaction back into the wallet).
Each time the wallet uses one of these 100 addresses, it generates a new address to be used later and adds it to the end of the pre-generated list.
This means, once you've gotten through 100 addresses, you will be using addresses that weren't included in the backup that you made previously.
It is also a good idea to keep a few recent backups just in case the most recent backup is damaged or otherwise unusable.
I generally suggest keeping an estimate of approximately how many new receiving addresses you've generated and how many transactions you've sent since the last backup.
Whenever the sum of those two values is approximately 25, create a new backup. Then keep the 3 most recent backups on 3 different devices in 3 separate locations.
That way, you'll have 3 copies that can all be used to recover your entire wallet if you encounter a problem, and you'll have a little tolerance built in if you count the addresses and sent transactions incorrectly.