I think this would be an excellent idea. The bad news bits: as far as I'm aware, none of the datasets that do things like match postcode to ward include polling station information. That data is all stored at council level, and so split amongst 400+ bodies. Polling station locations can also be changed at short notice and without any legislative process by councils, so it's also data that needs keeping up to date. But the good news: I think that the service would prove very popular with lots of other people and websites, so there's a high chance of all that work being worthwhile - and of getting volunteers to help. I think other people picking up on the service is what will make it a success rather than people coming to it directly. In addition to contacting MySociety people, it may well be worth contacting VoteWise (www.votewise.co.uk).