Saddle bag for food container I have a ~2 mile/~20min cycle to work and currently take a rucksack with me that generally only contains my food for the day in a standard take-away container measuring about 17cm x 12cm x 4cm give or take.

I end up with a sweaty back in the morning which is a little annoying; so I'm wondering if there are any good saddle bags or similar that I could fit my food container, wallet and phone in (so something with a depth of maybe 10cm) to relieve the issue of sweaty back.  Does anyone have any good recommendations because at the moment my rucksack seems a little like overkill.
It sounds like you need something like a handlebar bag or a rack trunk like these:

rack trunk

handlebar bag

The rack trunk would require adding either a seatpost mounted rack or a full rear rack. Either one would allow you to move to panniers in the future if you needed to carry more goods like a laptop or a change of clothes, but a full rack would be able to handle more weight and is generally recommended for stability and versatility.

The handlebar bag may or may not require a front rack, depending on how big it is. Some come with enough of a mount that you don't need anything additional to support the weight.