Thanks for the words of support everyone. I'll be fine, though I think I'd better move myself out of first tri! I shall ask the mods to move this to miscarriage and loss in case it scares anyone else!
Yes, I will have to go back for the second scan anyway - although the midwife started off by pretending it was in case it was too early to see a foetus, as soon as she realised I knew exactly what the situation was, she didn't try to pretend there was any hope. At an educated guess, I would think that the scan next week is to establish whether the sac is shrinking of its own accord, or whether it is still growing (since the hormones are released by the placenta, not the foetus, in the case of a blighted ovum the sac can carry on growing till past 12 weeks - at least I found out now). I assume that the results of the next scan will determine management of the miscarriage, ie whether to let it happen naturally or whether intervention will be required.
Whilst the sonographer was doing the scan, I guessed immediately what was up, and tried to catch the measurements as she was reading them out to the staff nurse. I didn't catch the third one, but the first two were 21 and 7. For a diagnosis of anembryonic pregnancy (blighted ovum), you would expect a gestational sac of 12mm+ and no yolk sac. The average of 21 and 7 is 14mm, obviously the third measurement could change things, but when I asked the midwife said the sac measured exactly 6 weeks (which is 14mm); I'm 6+6, though as I ovulated on CD18 of the cycle, I would have expected any scan results to show around 6+2. The yolk sac should appear in the 5th week of pregnancy, so there's a bit of leeway built into the 12mm measurement anyway (i.e. you'd probably see it when the gestational sac is around 8mm+).
Of course, there's always a little room for error at such an early stage - if the sonographer was just a couple of mm out, say - hence the need for a second scan for confirmation. However, it would take a remarkable combination of factors for the next scan to show any better news. I'm disappointed but not entirely surprised. Whatever the truth of the water/diet/morning sickness theory, I felt that something was not quite right, even though I've not actually been bleeding.
DH is hoping I get to keep the bigger boobs though...