I've had quite a few things (dresses, tops and trousers) altered at M II M (Made to Measure) in Jumeirah. they've always done a really good job and at a good price too.
Thereis a placshop in Mazaya Center , called Zinc . they have nice night gowns ans they also do alterations. I had a skirt made by them lastmonth and they did a great job.