Since introduction, has the Roomster back bumper stayed the same, or has it had any modifications, excluding whether it is fitted with reversing sensors.

No, I haven't bent mine, but my Bro in Law has apparently fubared his.

Sorry I can't answer your question but in June 2011, a new bumper for mine cost £140 + vat.  

Painting and labour costs to replace the bumper took the final bill to £380 + vat.

Hope this is helpful.

Thanks, it was a question from my nephew who will be tasked with the job of fixing it. He has located a 2007 bumper and the car is a 2011 (I think).