Having worked in a field that uses a silly number of technical acronyms, I was taught that a or an is used assuming someone will read the acronym in full, so a Marks & Sparks dish... makes sense
However, I'm pretty sure most newspapers and broadcasters assume you will say the letters as written so an M (em) and S, I think this might be an actual slight shift in everyday grammar, not just the difference between technical writing and speech.
Which means at the moment it's hard to be wrong
