The Cucurbits family includes Courgettes, Cucumbers, Squashes and melons.
Difficulty rating: medium
Suitable growing site: open ground or large pot in a greenhouse -  balcony, garden or allotment
Days from sowing to harvesting: 90 days+.
Risk of cross-pollination: yes, squash will cross pollinate with other summer squash.
When to sow: sow courgette seeds in april in pots of peat-free, multi-purpose compost. for later crops, sow seeds directly outside in late spring after risk of frost has passed. Pot on seedlings when they’re big enough to handle, and plant outside when all risk of frost has passed.
Requirements: courgettes are heavy feeders, so need soil enriched with plenty of well-rotted manure or compost. Water plants regularly and feed weekly.
Harvesting for eating: for courgettes, harvest while they are small (about 15cm long is ideal), otherwise leave to grow into huge marrows for use in stuffing and soups. 
Harvesting for seed: allow fruits to become fully huge, leave in place until the very end of summer and only harvest as the plant begins to fully die back.
Here's a video of Vicky explaining how to save seed from cucurbits: