Article 41, Sec. 3 of the Vienna Convention on Diplomatic Relations (and, depending on the degree of responsibility diplomatic staff have in contributing to Assange’s actions, possibly also Sec. 1 of the same article.)
> I can see them put in a situation where Britain doesn't respect Assange's diplomatic immunity,
Assange has never had diplomatic immunity; the issue is Ecuador allowing it's embassy to be used in a manner inconsistent with its diplomatic purpose, which, yes, is well past the point of an individual proposed diplomat being rejected by the receiving state.
From my perspective, it seems an embassy's right to hold a refugee was shown in the Noriega affair. If they are allowed to shelter a refugee without losing their diplomatic rights, I don't see how they can apply diplomatic law to him.
Allowing him to blog per se also not.
Allowing him to blog (and, even moreso materially supporting him in so doing) in a manner directed at influencing the internal politics of the UK, however, is, because such interference (aside from any participation in it being expressly prohibited to protected diplomatic personnel by Sec. 1 of Art. 41) is universally understood in international law to be incompatible with the function of a diplomatic outpost.
I'm also unclear as to what law you say they're violating. At most, I can see them put in a situation where Britain doesn't respect Assange's diplomatic immunity, and we are well past that point.