[quote FindTheTruth]**@rogdmum* Mermaids and Stonewall have been refused permission to get involved.*
So Mermaids and Stonewall applied to be a third party intervenor and were refused.[/quote]
This document about third party interventions gives a possible explanation for the refusal.
justice.org.uk/wp-content/uploads/2016/06/To-Assist-the-Court-Web.pdf
On page 6 they describe what is meant by a third party intervener (my bold):
1.11 An intervener is distinct from a party – whether claimant or defendant, appellant or respondent, prosecutor or accused. A party has a direct stake in the outcome of the case, whereas a third party intervener does not. Generally, a third party intervener should not be confused with:
(a)An interested party: an interested party is someone who is identified by either the claimant or the defendant as being directly affected by the case (in particular, the relief that may or may not be granted by the court depending on whether it finds for or against the claimant).
I think both Mermaids and Stonewall may have been identified as 'interested parties', rather than third party interveners.