"do" or "does"? (UK english)

I am currently writing a survey to gather information on the possible demand for a new community group in my area. It is a fairly wide area, so I need to include a question about postcodes. Whilst writing the survey questions, I wrote down "Which letters do your postcode start with?" but that didn't sound quite right to me, because I thought "do your postcode" sounded incorrect, so I tried "Which letters does your postcode start with?" but that didn't sound right either because "Which letters does" sounded incorrect, and starting the question with "What" instead of "Which" only made it more confusing. I also have to use the word "letters" rather than "letter" or "letter(s)" in the question because all the postcodes in my area begin with two letters, and knowing only the first letter would provide no information at all.

Does anyone know what would be correct in this situation?

Thanks,

Xander