Is there a circuit simulator out there that has latching relays?
@Andyaka Thanks for the tip! This seems to get me so close to what I
  need, with the exception of requiring two switches to control it. The
  functionality I need is a single momentary switch latching the relay
  on, then latching it off again on the next press. I imagine I'm
  missing something fairly obvious in making this happen, so will keep
  playing, but felt it couldn't hurt to ask for an extra prod in the
  right direction please?


OK, to make this work from a single switch, tie the D input to the inverted output of the D type flip-flop (Q bar) instead of tying it to logic 1. Now, each time the switch is pressed (i.e. a single press puts clock high and then returns it low when released), the output toggles state and remains latched until the next transition on clock: -



If you were to build it in hardware then a good trick is to incorporate contact debounce like this: -



Taken from this answer from 2013.