Earlier this year I was given a packet of cucamelon seeds, Melothria scabra, and not having a glasshouse (yet) I potted them up and keep them in a plastic box in the living room, where they are gradually growing. Whether they'll ever get big enough to plant out in the tunnel this year I don't know. To save time, as we're just going out shopping, I'll quote Wikipedia -
"Melothria scabra is a vine grown for its edible fruit. Fruit are about the size of grapes and taste like cucumbers with a tinge of sourness. Vernacular names include mouse melon, Mexican sour gherkin, cucamelon, Mexican miniature watermelon, Mexican sour cucumber and pepquinos.
As you might have guessed, they come from Mexico - and Central America. I'll keep you posted!