Hi it sounds to me like you just have a baserail left with the narrow groove size, I would suggest replacing this baserail that sits on the top of the stair string (stair side) with a new one with a standard groove size, or put a new one on top of the existing with a standard 32mm groove size...
Hi there is a range of stair coverings you can get, these are manufactured in Italy but there are people in the UK who deal with them like http://www.stairplan.co.uk
URL http://www.misterstep.com/kit_stairs/stairs_coverings.htm
Just an idea.
Regards
Lee