I don't think my post (or my thinking) was very clear, so it's my fault and not yours.
Having found the rules, it's clear that ST can stop the process dead by withdrawing their claim.
So now I understand that an intervener could not stop a settlement between the main parties, even if it was detrimental to them, except for trying to have a word with the respondent?
I think that's right. If the parties want to settle, they can. The claimant then withdraws their claim, and it's all over.
if it were not for SEEN Defra would/might have settled already
As, in, I suspect, if SEEN weren't prepared to kick up a fuss. That would be before applying for intervenor status. If DEFRA settled with ST very early on, there would be no court process in which to intervene. SEEN's hidden threat if SEEN were blocked is a tribunal case brought against DEFRA for discrimination on the grounds of belief, SEEN members now being the claimants.