How do you begin a sequel?

Without giving too much away, The Secret of Mhorrer starts five years after the events of The Secret War and includes a few characters who survived the first book. I hurl the reader straight in from the beginning, but while I hint at the exploits of the first book without resorting to a “story so far…” prelude (which I always find a little cheesy), the references to The Secret War are scant.

As an author of a sequel, should I make assumptions the reader will have read the first book? Or do I labour the introduction, painstakingly including references?

I don’t know many people who read things out of order, but I’m sure there are those who do. Critics and reviewers, for example, might not have read the first book, but might be asked to read and review the second – so what of them? Will they be flummoxed by the setting, the characters and the overall theme of the book? And what breadcrumb trail should I lay to ensure they know exactly what is going on?

I might test the second book on someone who has not read the first. If they can understand it – and even better – enjoy it, then I’ll have succeeded (I hope).
Otherwise…

It’s something I should have thought about during the first draft. Or even the second. I fucked up there – but I’m thinking about it now!

Answers on a postcard please.

Failing that, please comment below.