To quote from a pdf published on-line by the Flag Institute:
“Controversy persists about whether the British national flag should be called “the Union Flag” or “the Union Jack”. Among those who are informed about the matter, it is generally accepted that either name may be used. However, some people are convinced that “Union Flag” is the only correct term and that “Union Jack” should be used only when the flag is flown from the jack-staff in the bows of a Royal Naval vessel or, at least when it refers to the use of the flag by the Royal Navy. The general public almost universally uses the term “Union Jack”. Today some people claim to perceive that this usage is being eroded by persons who deliberately adopt an unjustified pedantry. It must be acknowledged, however, that the origins and relationships of the names “Union Flag” and “Union Jack” are historically far from clear“.
I suppose, as I have previously refereed to Union Flags hanging from buildings I have photographed on my travels, I should refer to this as a Union Jack, especially as it is being flown from a “jack-staff in the bows of a Royal Naval vessel” – but I could be wrong.
The M. 33 has made an appearance on the Antic Roadshow, back in 2014, so if you would like to see what it looked like then or learn a bit more about its history follow this link.