Slight leak from underside of basin waste

[shd1970]

I have fitted some new taps and a new waste to my existing bathroom sink.
Taps are fine, but i have a slight leak from underneath the basin.
It appears to be coming from between the underside of the basin where the rubber washer and plastic nut tighten onto the porcelain.
I did have a concern as the hole underneath in the porcelain did not appear to be consistent all the way around so i applied more of a liberal amount of sealant than would typically be necessary.
Looking more closely at it now, the shape of the porcelain has caused the rubber washer to bulge out a little so i think this is the cause.
I looked for a 1.5" equivalent of a shaped trap washer (see this link http://www.diy.com/departments/plumbsure-trap-washer-pack-of-3/178382_BQ.prd ) but could only find a 1.25" so just used the standard flat rubber washer that came with the waste.

I was mindful not to over-tighten the plastic nut by the way.

I am resigned to the fact that i will have to dismantle and redo but wanted to see if anyone can offer any advice as to how to prevent this happening the next time please ?

FYI, this was the order from top to bottom :

Waste
Foam washer
Bead of sealant
Top of basin
Underside of basin
Sealant
Rubber washer
Plastic nut

Many thanks in advance

 

[Newboy]

Try one of these - http://www.toolstation.com/shop/Plumbing/d20/Washers+&+Fixings/sd2703/Basin+Mate/p37534

 

[shd1970]

thanks newboy
DAMN....just realized that the original waste was 1.5" where as the new one is 1.25" !!!!

are my eyes correct newboy.....is the shaped rubber washer threaded ??

 

[Newboy]

Not threaded just ribbed - forms a very tight seal on the threads which is where the leak usually comes from. Basin wastes are always 1 1/4" - sink waste are 1 1/2". (you don't have a sink in your bathroom, you have a basin - sinks are for kitchens!)