If you shared a common garage wall originally it will be very obvious from the line of his original exstension whether he extended this line (which I will 99% guarantee would have been shared and on the boundary or not) or whether he built inside the line on his side. If he built inside the line, you cannot expect to use his wall as a common support for your roof. You will have to build another wall right up against his. The issue you are going to have to solve is how the roof will work - if his is a flat roof, I hope the slope is to the rear of property and also that you can do likewise. Even then you will have to come to an arrangement about joining the roofs together as you clearly can't have a gap.
None of this changes the fact that the cowl for his CH comes through this wall now, and therefore protrudes onto your land.
There will be nothing in your Land Registry documentation about this - the Title Plan will merely show the red marking around your plot and presumably the boundary at the original garage join.
The party Wall Act should have applied to his extension (you had some rights - but it is too late now) and it will apply to your Works. The Party Wall Act will allow you a legal access onto his side to effect the extension, require you to clear up his side after you have finished, make sure you design your foundations so that they don't undermine his, that sort of thing. Read the link I gave you.