It takes quite high doses of vitamin D to correct a deficiency, so if you were taking the 200IU or so in a cod liver oil capsule, it wouldn’t do much for you. The serum test is cheap as far as private testing goes (£30 for a home test kit), but maybe your doctor could tell you if it’s been checked already. I get a feeling they don’t check serum levels, even though 1 in 4 people in England are deficient.
It’s really something your doctor should know if he’s picked up a journal in the past decade, e.g. this review of MS and vitamin D. You might have read about that guy campaigning for vitamin D fortification in Scotland to combat the high incidence of MS there too. This review from a nutrition journal gives a good overview of how widespread the problem is and the consequences of a deficiency. Hopefully your doctor isn’t useless and your levels are already stable.