Any chance of a photo, Colin? It sounds like a fungal problem. If the spread from foliage to stems to fruit has been rapid, it could be Late Blight. If not rapid, it could be a number of things. There are even fungal problems specific to greenhouses that are rarely found with outdoor toms.
Anyway, given that it's probably fungal, you can never guarantee against it ever happening. Fungal spores are airborne, they travel on the breeze, they're everywhere in the air, and invisible to the naked eye.
One option is preventive spraying, which means spraying before the spores arrive. Once they have arrived - when the symptoms are starting to show - it's too late. It means spraying about once a week from not long after the toms are planted out.
If you don't fancy spraying, you have to concentrate on a housekeeping routine that aims to minimise the fungal spores' chances of getting a grip.
In a greenhouse, ensure as much air circulation as you possibly can. The closed environment can be an incubator for disease.
Keep as much space between plants as you can for air circulation purposes.
Try to avoid great clumps of dense foliage developing on plants by judicious pruning of excess foliage and branches. Again, for air circulation purposes.
Keep a gap of at least a foot between the lowest foliage and the soil. Fungal spores can and will fall from the foliage to the soil and can be splashed back up again when watering causing re-infection. The gap will help against this.
The bottom line is that you can't avoid fungal spores. All you can do is try to minimise their impact.