What proof of English do I need?

Short answer: Two layers. The call sets what the committee wants, often a certificate or an interview in English. The visa layer is stricter: the national rules let embassies use only certificates showing at least B2 in all four skills, from internationally recognised bodies or from universities (MUR circular).

• From Dhaka, the embassy's checklist accepts only IELTS for English, at least 6, issued within two years, unless the university records your English on Universitaly (Dhaka checklist).
• An MOI letter, even from a strong master's, does not test four skills. Take IELTS early; it also removes doubt in the interview.
• If you studied your master's in the UK or another English speaking country, ask your university whether it will certify your English on Universitaly.