Thanks for the tip, Siduhe. The perpetrator of the crime against enamel has contacted them, and they are coming next week to repair the chip. The quote is pretty expensive, but I don't care as I'm not paying (in fact, I'm pretty pleased it's expensive - that'll teach him for chipping my bath. Ha!) I'll report back on the quality of the repair. If the repair isn't good enough then I shall be demanding a new bath!