Stats thread deffo a good idea, missC - not sure how I missed your post on the previous page but I did! I'll head over there to add myself.
Marff lies! I am not a dealer! by the way, you still owe me for the last fix and if you don't pay by sundown the heavies will be 'round, right?
In all seriousness I am an advocate of high dose B vits, and despite there being no 'official' studies (because there's no money to be made from vits for a pharmceutical company, see?) here's a 0-60 of why:
High dose b vits have a positive effect on the following: lengthening luetal phases that are short, or on the shorter side of normal. Good because it gives any potential baby the time it needs to implant before getting flushed away by af turning up too early. They also seem to balance the cycle more generally - so if you O on the late side, you may find this moves forward by a few days.
Hair, nails and skin will probably improve too.
There are a couple of studies with IVF patients that find if the women take high dose b vits in the follicular stage of their cycle that they produce 'better' eggs. With ivf, when the eggs are retrieved, they are graded, with only the best ones being used for treatment (although if a woman only gets fair eggs they may go ahead with those anyway...) I find this fascinating and to me it's a bit of a no-brainer that if you can improve the quality of your eggs (and therefore improve your chances of a outcome) with a simple vitamin pill then why wouldn't you?!
Many people (there are men that take high dose b vits too) report that they just inherently feel better - they apparently have a positive effect on low-grade depression and stress. Let's face it, most of us suffer to some degree with both following a loss and so potentially reducing the effect of that, again, can only be good (especially wrt ttc again - stress and/or depression doesn't usually lead to much swi, right? And can affect your cycle negatively too, usually with delayed O or no O at all).
As I say, this 'evidence' is very much anecdotal (and my own experience... plus just about everyone I've pushed to recommended take them seems to have a positive story to tell) but here's the kicker: In the face of so much positive anecdotal evidence if it were a herb/medicine/something with 'danger' attached then you'd think carefully... but because your body will take massive doses of B vits before it shows any sign of 'overdose' even the 'high dose' (compared to rda) is in no way dangerous. B vits are water soluable and so whatever your body doesn't utilise will be pee'd out (and give you bright yellow/neon wee - don't worry, you're not suddenly radioactive, it's absolutely fine!).
So, that's about it, in a nutshell. No need to take my word for it, there are lots of stories on t'net if you wanna research further 
Oh, apart from where to buy. I've always gotten mine from holland and barret - they often have an offer on, on one or the other two bottles they sell (one with fifty capsules, the other with one hundred).
Happy now, marff? and PAY UP!