Here's what I did...
First - I forgot to take a photo of just the plain stains... but you might be able to see the blacky greasy stains that I haven't painted detergent on yet in this photo...
It did get rid of a lot of the stain - the sort of greasy patch but not the blacky burnt bits from the BBQ splatter. So I got some prewash stain remover and squirted it on for the second and third go. I left it on for about 10 minutes each time (didn't want to end up with white spots instead of brown!) and soaked it on warm water again.
After 3 goes - the stains were pretty diminished but not entirely gone. I wore it to work, I work with kids and come home pretty covered in grot anyway, but wouldn't wear it out to lunch for example now. Apparently there are lots of cans of goop at the supermarket that will help get stains out. They are all solvents of one kind or another. At this point I'm going to dab it again next time it goes into the wash and wear an apron if I go near the BBQ in nice clothes again!!!
I have got a book called "how to clean practically everything" and have consulted it - It says to try white vinegar or lemon juice for greasy stains, again dabbed on with a paintbrush - so next time its due for the wash - I'll give it a go and let you know what happened!
Green-ness: 3/5 Green detergent is ok but I suspect my prewash spray is pretty toxic. I comfort myself with the fact that we've had that bottle for about 2 years... Maybe that's why it didn't work...
Frugal-ness: 5/5 Didn't spend an extra cent and can still wear it
Time cost: Most of the morning - but I was making bread and pottering around in the kitchen anyway or maybe that's why it took so long?!
Skill level: Easy-Peasy! Just do it!!!