Does this help....276 parts diagram on Hitchcocks websitehttps://www.hitchcocksmotorcycles.com/p ... pages/3379
Re the leak...if petrol is coming out of the notch in the body just above the union nut as you say that's the overflow.
The needle seat is part of the float bowl...I don't think you can get a replacement seat. You can try lapping the cone of the needle into the seat with some autosol but go gently as its only brass and try to keep the needle upright/central or you will lap an oval into the seat.
Is yours a bottom feed like the one in the diagram? If so as you say you can take the float chamber top off and check the needle valve is sealing...attach the fuel pipe and turn the petrol on. Float should rise and then the valve should seal.
Make sure the clip that fits on the needle above the float has not come out of its groove and is a tight fit.
Make sure the 'tickler' is not sticking down slightly.
Re fitting thicker/ dowety washers to the float chamber connection remember that altering the height of the float chamber relative to the carb body also alters the fuel height.
Because the float chamber connection is at an angle you can rotate the float chamber fowards/backwards which also slightly lowers/highers the float chamber, again altering the fuel level.
Best of luck.
If you do a google search on amal 276 float needle seat forum
there is loads of info on the subject