John,
Should be a formality, however, your fiancee may be required to undertake a medical, Im not sure whether this happens at Manchester, but it is something that happens at Heathrow, health screening is common now, most of our partners managed to escape it, I told the immigration officer, that my wife was pregnant, and she let us go through without any problems, you can always try that, and say you think she is pregnant.
The downside of being asked to attend for health screening, is that if their are other people waiting to see a Doctor, it can slow your progress through immigration, but you will just have to suck it and see, I take it she is coming unnacompanied, so there isnt much you can do, just tell her, to say she thinks she is pregnant, and they will probably wave her through.
As previoius poster has said, just the passport, and you being available to collect her..apart from that, nothing else to worry about.
Good Luck John, and I suppose you will be pole vaulting through arrivals leaping over tall buildings and climbing large mountains....enjoy ka !
