Lpc2000

>> What toolchain are you using ? <<

Crossworks.

>> Depending on toolchain, I could send you the modfied startup file  
directly.. <<

The datasheet says:

"The reserved ARM interrupt vector location (0x0000 0014) should contain the 2 
s complement of the check-sum of the remaining interrupt vectors. This causes 
the checksum of all of the vectors together to be 0."

So, I'm thinking it shouldn't be that difficult to actually work out, but I'm 
don't understand whether it's just talking about the first few memory 
locations, all the interrupt vectors, both, or something else again. 

I should work out how to calculate it really, as there's every chance I'll 
want to mod it again!