Find numerical difference between text values.

Hi all, first post here.
I've been asked to do something in excel i can't seem to get my head around.

My client provides me with data in the form

1c, 1b, 1a, 2c, 2b, 2a, 3c etc... where 1a is the lowest.

I need to find a way of working out the difference between these, e.g.
edit: 2a - 1c = 5

I've been working on it and have made some headway by splitting these with MID function and then using CODE to try and work out a unique ID for each, but i can't help feeling there's an easier way!

I'd appreaciate any help or suggestions.

Many Thanks
Matt