Just tried adding the "customers also bought" block to the basket page - that seems like a really obvious place to put it - but it doesn't work...?

I added it to the centre block of the basket template and nothing appears on the basket page - what's up with that?

Amazon has been bombarding us with suggestions and "also bought's" during the basket/checkout process for decades, and they're still doing it. Given the amount of data and analytics they have, and very smart people testing and analysing everything that implies it's an effective method of increasing sales.