Party wall with Flue

[WhyBother]

Having moved into a linked detached house two years ago I share a garage wall with my next door neighbour, he has extended on his side into the back garden to creat an extended kitchen.

I would like to do the same but he has placed the flue for his central heating boiler through his newly extended wall that is in line with the previousparty wall on the property boundary, thus pumping his gases out into my garden.

This prevents me from extending as his flue would vent into my kitchen if I extend in the same way as he has, what are my rights? Can I insist he moves his flue?

I have reas the party wall agreement and there is no mention of his extension, nor is there any apparent amendment since the house was built in the 60's, prior to his extending.