Reading between the various lines, I think Birch is aiming to call into question the validity of Chainrai's £17m 'debenture', isn't he? Thus wiping out a significant percentage of the debt in one swish of an email.
If there is no tangible / legal evidence that Chainrai (Portpin) loaned this money to PFC (as opposed to giving it to them outright) then I can see Birch turning round and saying: "ok Mr Chainrai, sue us if you think you have a claim to that money."
Whilst one assumes there is evidence that £17m went from Portpin's account to PFC's account, is there any evidence, other than anecdotal, that it was a loan and not a 'gift'?