Can I .......

swop creme fraiche for sour cream? I've got a recipe for a cinnamon loaf which needs the former and I've got the latter  It's a cake loaf as opposed to a yeast one. Thanks as ever lovely people, Jude.

This site  http://www.ochef.com/100.htm
says

Ochef:In general, crème fraîche and sour cream can be used interchangeably in most recipes, but crème fraîche has two advantages over sour cream: it can be whipped like whipping cream, and it will not curdle if boiled.

there is slightly less fat in sour cream so don't scrimp on the butter/margerine/oil in the recipe, if you do decide to swap

My friend adds some lemon to cream or creme fraiche and that makes it sour give it a  go and you will have what you want.