Sorry, Goldenoldie, I didn't mean to put you off going to St Mary's - congratulations on expecting twins!
My anti natal care at St Mary's was fine, although as someone else has mentioned you do wait a very long time to see someone when you attend the anti natal clinic. I actually spent a lot of time on the labour ward from 26 weeks onwards as I used to get runs of BH contractions on a regular basis and I was never sure whether they were the real thing or not. I have no complaints whatsoever about the care I received on the labour ward. I ended up checking myself in at 3am on several occasions because I couldn't sleep as the contractions were so strong! I was always seen by a midwife pretty quickly although it often took longer for a doctor to appear.
One thing that did bother me about Mary's is that although their SCBU is excellent it is almost always full. This means that if you do go into labour before 36 weeks you may end up being transferred to another hospital. However, I think this is pretty much the case for all of the large London hospitals. When I was 29 weeks, everyone really thought I was going into labour and I was almost transferred to a small hospital in Essex as there was no spare intensive care cots anywhere in London - a shocking situation.
I know several other people on this thread have complained about the post natal wards, however, I'm not really sure it's any better elsewhere in central London. Certainly, friends of mine have had equally bad experiences at the Royal Free and Chelsea and Westminster.
By the way, if you live anywhere near Queens Park, some friends of mine run a twins club on Thursday afternoons at the Hopscotch nursery. If you'd like any more info about hospitals/twins etc do come along. I think almost all the twin mums who live near me had their twins at St Mary's, and while everyone complains about the post natal care, I don't think I've heard any complaints about the labour ward which is obviously much more important.
Best of luck!