The rule, as I understand it, is that first names don't take the "s", unless they are foreign, but surnames do. The problem here is that saints don't have surnames, so there is room for argument. The football club don't use the "s": http://www.nufc.premiumtv.co.uk/page...,10278,00.html
I suppose the correct form would be the one used by the owners in these cases where there is room for disagreement.
I'm a bit confused now. You said first names don't take the "s" (which implies that the first name is the criterion). Then you said the problem here is that staints don't have surnames (which implies that the surname is the criterion).
Suppose there is a Mr Thomas Davis.
Would his close friends say: Thomas' house OR Thomas's house
Would others say: Mr Davis' house OR Mr Davis's house
Acording to the 'rule', it'd be:
Mr Davis's house
With St James, where we don't use a surname, we have a sticking point. As you can see, Newcastle do one thing and the royal palace do another. Pedants would probably say that the royals were right. The problem with a saint is that 'James' is the only name- so it's not really a first or a surname.
I tend to think that saint names are first names. Because we have St Peter, St Paul, etc., and these are never a surname, aren't they? Thus, I think what Newsastle do is right and the royals are wrong.
In all honesty, I'd leave the "s" off, but for the fact that they don't and I used to work in a college there. If this was used in the spelling contest, then it's a bit unfair IMO. If I were naming it, I'd say that it was a first name and follow the football club. But then, as we know, royals do things differently.