Without having seen the jacket it does sound like it's too big to be taken in decently, clothes lose their shape if you take them in too much. Does it have lining??? You could ask at your local laundry they sometiems offer alterations.
I've just asked my sil and she said it's probably best to either look in the yellow pages for a seamstress or if you are in Manchester to go into Granada Dry cleaners in town, North John st I think and they might know of someone who can help. She said companies like Johnsons will probably say it's too much work and be too expensive if they did take it on.
It is a job for an expert. Only trouble with that is that it will be pricey and you dont know how good it will look until its too late.Is the length ok? because if that needs altering too then I dont reckon it will be worth it.
How about selling it again on ebay and buying one thats a better fit? That might be quicker too.