Autosomal dominant diseases arise when somebody has inherited a mutant form of a gene which codes for a protein which has a pathological "gain of function" effect.

They are very rare and show a very strong genetic pattern. If your SFN arises from an autosomal dominant effect then one of your parents would also have SFN as would one of their parents and so on. The fact that this is not part of your family history means that an autosomal dominant effect can be confidently ruled out.

The MTHFR gene effects which mrsD has mentioned are different. Mutants of this gene (there are a couple of common ones) lead to its encoded protein having an impaired function. This can have many effects and is certainly worth checking out - the Oxford geneticist should be able to advise you about this.