Deleted wallet and missing coins Sequence of event:

a) backup of wallet created
b) created new receiving address
c) coins tranferred to new address
d) wallet deleted

I've restored the wallet from (a) - but it does not have the address (created in b) I'd transferred the coins to. Can I assume the coins are now gone?

Thanks!
Coins are gone when you can't recover your private key.

If you have used a non hd wallet (and i think you did), you can check if your bitcoin wallet supports keypools. Keypools are a set of pre-generated keyprairs (pub/privkey) which can be used when a new key is required. Because generating a key (non HD) in a encrypted environment needs to unencrypted (you would need to enter your password), keypools make sense in non-hd wallets.

With keypools (and i think you used a non-keypool wallet) there is a chance that your receiving address's private key – which you used after you created your backup – is still on the disk.

Last chance you have is to try to recover your deleted wallet file. Therefore best you could do, stop using/writing on the disk where your wallet was stored. Filesystems mostly don't really delete files (write the bits to 0), they mostly just allow other data to be written in that space. Because of that, you can use undelete software to try to extract deleted data from your disk. You should try to tell your undelete software, how your lost wallet file does look like (files magic, file structure) and this could be hard and probably requires a professional.

There are always chances to recover data and i hear of people successfully recovering data from physical damaged hard drives (fire, etc.). It always depends on how valuable the lost data is/was. A data-recovering in the laboratory can cost around 1000USD.

Next time it's maybe worth to use a HD wallet. During the creation process you can write down the master seed (sometimes a hex string, sometimes a wordlist, depending on the wallet).
If using HD, this master seed technically allows you to recover all your possible private keys.