But even still, "the body" is an odd choice of words if they didn't know for sure Becky was dead. If they'd said "a body" or "Becky's body" it would have been more ambigious. "The" suggests to me that the sender expected the recipient to know exactly what he meant, and even to know it was about Becky specifically.
All that said, I still don't think any of the 4 of them knew for sure they had body parts in the shed. I think NM was a dealer (to whatever extent) as the offer of £5000 could only really apply to that sort of shenanigans. I think they must have had massive suspicions but they didn't act fast enough - had they called the police right away they'd have been exonerated by now. I reckon they just could not believe their mate could be a killer (I mean, who would?) and figured they needed to help him out with his dodgy dealings, stay quiet, get their cash, and continue on as normal.
Hmmm. I'm conflicted with myself. I'll go with: didn't know initially, suspicious towards the end, pretty damn sure at the last minute, didn't do the right thing and call the cops. Sentence? 5-10 years each for being so foolish, on all counts.