I sowed some outside a good few monthsago before the wet spell with no special treatment (compost, feeding, watering etc) and they're weathering the heat quite well.
The back of the packs (at home) say that they can be sown from July to September successionally. They need to be weed-free to flourish. I would have thought that in a greenhouse environment they'd do quite well provided they don't dry out when young. I'm going to sow some tonight (in the ground) and provide sun shelter and a water trough to the side and will try to remember