Am I correct in thinking that it's no longer possible to upgrade to a jailbroken iOS 4.3.3 (assuming I don't have SHSH blobs, which I don't as I've never been on 4.3.3 before) ?
You don't have to be on any particular firmware to get your SHSH blobs. I'm on 4.1 but Cydia has my blobs for 4.1, 4.2.1, 4.3, 4.3.3 & 4.3.4. If you've never been jailbroken and never told Cydia to save your blobs then you're out of luck. Best way to find out is to open Tiny Umbrella and run it and it will tell you what you have on file.