Slight shaking of the steering wheel at low speed?

Firstly, i have a 1.9 Diesel Ford Galaxy 52 plate.
as the title says my steering wheel shakes at low speed, with this in mind i fealt that it would be worth getting the tyres and balancing looked at so that i could eliminate these factors. The tyres are not out of shape and yes the balancing needed doing but this is only something that would and did make the steering shake at high speed. Needless to say the high speed shaking has now stopped but i still have the shaking at around 20mph goes after about 30mph but i can still ever so slightly feel it.
could anyone shed some light on what this could possibly be?

Easy. Tire balancing. Believe it or not.
Take your car in to get your tires balanced, something may not be zeroed in, causing your wheel to vibrate. It doesn't matter what speed you go either, fast or slow, if your center of balance is off on your tire, it could cause a vibration.

You might never know, your tires may be wearing unevenly and if they do a tire rotation on unevenly worn tires it could cause all kinds of weird things (like pulling, shaking, etc).

***EDIT: I read the question again, so you DID take it in for a balancing. Something may have not been balanced correctly? Or there's some uneven tire wear going on. Regardless, I'd take it back and tell the mechanic, maybe something's loose, causing a rattle until the tires gain enough centrifugal force to keep themselves totally upright? (like a gyroscope)