Went to see my physio this morning and she confirmed that, as I suspected, I have Plantar Fasciitis
She did some ultrasound and massage and has given me some specific exercises to do and said that I'm fine to use the cross-trainer but no running for the time being. She also suggested that I wear my running shoes as much as possible as they're really supportive so will help. They are very comfortable but wearing them does rub salt into the wound of not being able to run!!
She didn't want to be drawn on when I might be able to run again, but I'm going to see her again in a fortnight so will ensure I do my exercises very diligently between now and then.