Girl I like said she'd go for dinner with me and now has made a pathetic excuse

And dancing around the issue, possibly being seen to lead them on is in some way kinder?

It's really formal.

Staying in, watching films and eating Pringles with a salsa dip is a far better first date.

No, obviously not kinder, just the instinctively less awkward and painful thing to do at the time.

Maybe the painful/direct route is better. I know it helped me a lot when I asked out a girl and gave me a straight no. The world didn't end, everyone didn't turn to laugh at me, I would say the direct route is better and may help them.

So what would you recommend saying to a guy if he asks "do you want to go for a drink sometime?" It's hard to think of a good response when you're on the spot like that. Also bear in mind he might be asking in a friendly way and I'd be scared of jumping to conclusions and assuming he was asking me out, and then seeming big-headed if he never meant it that way.

Gotta disagree there, films are inherently antisocial, not great for trying to get to know someone.

Glad it worked out for you, OP!

Two things to people saying the girl (had she not been interested) should have immediately said no:

1) I don't know about other girls, but personally I don't always know on the spot whether I'm interested in a guy or not. I don't sort every guy into "would date" and "would not date" categories, preempting the moment one might ask me out. In fact, unless we're very close, it's not something I think about much at all and my opinions are pretty fluid. If someone asks me about, I really might not know if I'm interested or not. I might initially say yes, just to have more time to think it over.

2) It's not always obvious exactly what constitutes asking someone out. Okay, dinner is quite formal, so I think that would count as a pretty forward move. But when you ask a girl out for a drink, or to go to the cinema - what is that? I don't think they're immediately obvious as either a romantic or friendly invitation - I've done those with guys in both capacities. So it might be that she didn't really understand at first that you were asking her out that way and only suspected later.