I alwasy wondered if Sue Hercombe was his girlfriend, or just a "friend"
the oldfield book explains that DTTW is about a girlfriend in his youthness and that3 songs from he 1st album are about his 1st wife (six blade knife, water of love, setting up).
So if Mark had had a "serious" relation between 73 and 77, I guess he would have wrote a song about it
the separation with Holly Vincent was CIRCA fall 79 I guess. Ed explains in the Oldfield book that Mark sung WDYTYG like he never did before, the day they broke up, and it was during the 2nd US tour in 79.
I have a therory that his neck lace we can see on several pictures from late78-79 (e.g. on live at the BBC cover) was a gift from Holly, but it's only speculation
during the LOG tour, the Oldfield book talks about band members' "girlfirends" , including Mark, and at the time he wasn't with Lourdes