I guess my first questions would have to be ... can you list the ingredients in the gravy recipe? Without knowing what flavors are in the gravy it's hard to guess if mustard in the mash would be too much.
While not exactly "bangers and mash" in the English style - I did grow up eating grilled sausages and mustard potato salad (mashed potatoes, mustard, mayonaise, pickle relish, diced hard boiled eggs, diced onions, (one aunt always threw in some small-diced celery and pimento), a splash of cider vinegar, and sometimes some crumbles of fried smoked streaky bacon). The potato salad could be served either warm or cold - and we never put gravy on potato salad.
"It ain't what you don't know that gets you in trouble. It's what you know for sure that just ain't so." - Mark Twain