"Doesn't the evaluation version generate code which starts at 8000h?"

Yes, it does:
http://www.keil.com/demo/limits.htm
In other words, it's doing just what it says on the tin!
It always pays to read what you're getting before commiting to it!
caveat evaluator


"If so wouldn't this be greater than the 2000h capability of the 89C2051 CPU?"

That'd explain why the hex worked, but the 'C' didn't.

Anyway, as you have uVision you can still check your code in the simulator!

You'll have to get a chip with a suitable address range (probably external memory) to actually run it.

"learn assembler ... It's totally ... free."

Only the free assemblers are free, and usually still have some limitations; eg, only 1 source file; no commercial use;...
For an industrial-strength, professional assembler, you'll still need to pay!