The chanters are tuned G A B C# D E F# G A but C# is flatter than the tempered scale and the A is close to B flat.

Not being Louis MacNeice I know absolutely nothing about bagpipe playing but I suppose that to play in C would make you less out of tune with instruments in standard tuning than in any other key. (Though it'll surely still sound crap).

I suppose somebody Scottish (or Spanish) will be along in a minute to explain how to get round this (other than never playing C# or A).