This should have been pointed out to you when you purchased the property and your solicitor would have dealt with it, maybe a word with your solicitor, if he is friendly, would be appropriate.
On the other hand, I personally would approach your neighbour in a friendly way and inform him that you have just noticed that his shed is protuding into your property and ask if he would have it moved. I would offer him assistance but would not to contribute to the cost.
Regards
CSG2306