found 400 quid of clicks on a term i already had as a negative. how

It did exactly what you asked. You asked for the wrong thing, and this catches everybody once. Negative keywords have match types, same as positive ones, and the default when you add one quickly is exact. An exact negative blocks that phrase and nothing else. "hvac jobs" as an exact negative blocks precisely "hvac jobs" - not the singular, not the same words with anything added, not a misspelling. What you wanted is a phrase negative on "job", which blocks any search containing that word in that order, singular and plural handled by close variants. One phrase negative on "job" would have caught all four of the terms you listed and the twenty you have not seen yet. The rule I use: negatives that describe intent should be phrase or broad, because you want to catch the whole family. Negatives that describe a specific product or brand you do not sell should be exact, because you don't want to accidentally block the neighbouring term that you do want. Also worth knowing: negatives don't apply retroactively and they don't handle misspellings the way positive keywords do. If people are misspelling their way past you, you have to add the misspellings.