Then I read the squash seed packet " ...attractive cream coloured skin with dark green stripes....fruit is 18-22cm long by 10cm wide"- hmmm - pretty much exactly the fruit on the left!
This morning I microwaved it, and yes, it is a lovely squash. Slightly drier than a pumpkin, sweet but not overly so and quite nice eating. Reminds me of sweet potato in Timor.
So, the "watermelon" mystery is solved. There are two vines growing under the corn and the second one hasn't fruited yet, perhaps that is the watermelon? Will keep you posted and enjoy the squash in the mean time.
Squash 'Delicata'