It's my greenhouse, technically speaking not on the allotment but in my back garden. I did experiment with raising seeds in my allotment greenhouse, but I couldn't reliably get up there often enough and lost a lot when the weather turned warm.
So these days I sow dozens of trays of seedlings where I can keep an eye on them - as you can see, it's almost full and the sowing season hasn't really got into top gear yet! I have no idea where I'm going to put the seeds I intend to sow next week. I've already decided to direct-sow more than I usually do this year - leeks, for example, carrots and broad beans have been deemed tough enough to fend for themselves out on the plot. But most veg I raise in modules simply because that way, you can stop most things eating them, keep an eye on watering and growing rates and generally mollycoddle them until they're big enough to look after themselves.
Another somewhat overlooked benefit of greenhouses is that you can retreat in there in rainy weather, so you can carry on allotment gardening even in the worst of downpours. Essential equipment, I'd say - I just need a bigger one!