I have found a solution using the suggestion of rotating the photo 90 degrees. It works, but is a bit long winded to achieve the effect.
1. Select the landscape photo you want to crop to portrait.
2. Select crop and select square from the crop constraints.
3. Click on done.
4. Select crop again and rotate the cropped photo 90 degrees.
5. Select the crop size you require from the options in the crop constraints, for example 3x2.
6. Click on done to save the crop.
7. Click on crop again, then rotate the photo until the photo is in the correct portrait orientation.
8. Click on done to save the rotation.
As you can see, a few steps to carry out, but you can crop a landscape photo into a portratit photo. It would be much simpler if Apple changed the options available in Photos for iOS to include a portrait or landscape crop, just like Photos for Mac.
Hope this helps somebody else who wants portrait photos from landscape photos.
Schof