Thanks for the comments and suggestions, all
I'm not 100% where this is going yet - I'm actually only 37.26% sure for those who like statistics - so I don't have firm plans. In truth I've not really done any programming
whatsoever since I learned BASIC on a Vic=20 waaay back in the early 1980s, so I'm very much just trying ideas out and seeing what does/doesn't work at this stage. But to address a few comments and suggetions...
Pathfinding - not done yet but I'm currently knocking about a few ideas around Dijkstra/A* with various heuristics. Should be pretty straight forward for player pathfinding as long as I use tiles with "weighted" values. For Mobs/Enemies I'm interested in doing something a bit better where they don't pick an absolute path to the player but rather they investigate evidence of the player (for example, they'll head toward a square that's lit, even if the player isn't directly visible).
Movement - having continuous movement onto the next square (and the next, and the next...) if you keep the mouse down might be a nice improvement, so thanks

The player walking backward when you click behind is a concious design choice so you can back-off while still facing something, but if it needed changing it's take literally seconds to change it.
Random Map Genetation - This is something I'm really interested in , but for a different idea I have (I don't want to do another RogueLike - not because I don't like them, but more because there are already hundreds of excellent ones out there). I have, however, been faffing about with some terrain gen stuff based on cellular automata, as in the unbelievably basic tech-test below, with the eventual aim in mind of doing an exploration game.

Zipped demo attached to the post for anyone who's interested in that sort of thing ( left/right cursor keys to move, space to jump, click blocks twice to 'mine', up-cursor if you get stuck in a hole somewhere)
Space Hulk
Yup, love it and have thought quite a bit about doing something similar. The problem I actually have with squad games right now is the lighting setup in the game is surprisingly processor intensive, so large squads mean the game grinds down to single-digit framerates on phones. I'm working on it..
But, if you consider Space Hulk to be the movie "Aliens", I'm maybe more interested in doing something more suspenseful along the lines of "Alien". I like the idea of a couple of characters being stalked by something skulking in the shadows *shrug*.
Incidentaly, there's a decent Space Hulk -esque game on android -
https://play.google.com/store/apps/details?id=com.tresebrothers.games.templars&hl=en - called Templar Assault. Graphics aren't up to much but it's pretty good otherwise.
Anyway, I've waffled on far to much. I'll have a better idea of which direction I'm going in once I've mocked up a few more tech tests to see what can or can't be done.