Some reading... and some quotes.

In the case of a blighted ovum:
Typically the trophoblast and gestational sac will develop but the embryo itself will not - there will be no fetal heart beat, or it will be present only transiently
None of this tells us anything about the reason the pregnancy has gone awry, because many different causes produce the all too common picture of such a blighted ovum. Once the symptoms and signs of a miscarriage become apparent, it's for the subsequent pregnancy that we can hope there will be a difference
From "Getting Pregnant - A compassionate resource to overcoming infertility and avoiding miscarriage" by Prof. Robert Jansen.

Definitions from the glossary of said book...

Blighted ovum: An old-fashioned term for an inevitable miscarriage, meaning that the ovum (in its classical sense for professional ambryologists) has not developed normally after fertilization, there being present just the supporting tissues and no embryo. The term is descriptive; it has no diagnostic value as to the cause of the miscarriage.
Missed miscarriage/missed abortion: A miscarriage that should have happened but hasn't. As a result, the pregnancy tissue in the uterus gets tougher and more difficult to get out with a uterine curettage - which is the only treatment for it, but risks producing intrauterine adhesions. The pregnancy test can stay positive for many weeks or even months.
So... I guess that means it was both. Whether it was one or the other, it makes no difference to the cause, nor does it give any indication of whether it is likely to happen again. And it doesn't change the outcome for me now.

It does tell me that I'm not going ahead with another transfer until there has been investigation into the cause of this one. Whether it's done by the current FS or by Dr S, or both... it will happen!

BW