I have a small radio which uses a BL-5C , 1000 mAh 3.7 Li-ion ReChargeable battery. If i used a similiar BL-5C battery with 1020 mAh instead of 1000 mAh would it make any difference or would it mess up my radio?
No problems at all. The battery would simply last marginally longer (although, as there would only be a difference in the order of just 2%, it's unlikely that you'd actually notice it!)
Just to explain...the mAh figure is just a measure of how much power the battery holds. So, if you ever need to replace the battery, you could get one with a much larger capacity (bigger mAh figure) and it will last much longer between charges.
You might even find that you can't buy 1000mAh versions of the battery nowadays. As battery technology improves they manage to cram more Ah into the same physical size battery, so that's what they do and you get a longer lasting battery.