Mongoose are trying their best to make the pain and disappointment less. They are offering two options to Mega Deal customers:
1) A store credit at the full value of the Mega Deal you purchased for products now or later - ie, it's a store credit that has no expiry on it and can be used a little at a time.
2) A pro-rata'd refund based upon the amount of books outstanding. So, if you went for the £400 MD option then I'd have thought you'll get refunded somewhere in the region of £185.