I also learnt it the hard way.
Builders are not to be trusted , everything i mean EVERYTHING should be on paper.
It is generally true to say a verbal agreement is not worth the paper its writen on.
It has been my experience that when dealing with builders, trades, developers including Architects, Engineers and the like, it is best practice to confirm all your agreements in writen format.
Your first course of action should be, confirm too the builder by registered post, a non-emotional, no frills, factual letter, stating that you are not willing to except his contract variation. This is mainly due to inadequate information and due time for consideration prior to signing the contract.
You then request he proceed on the bases of your prior agreement, mentioned in said contract.
Followed by you clarifying the agreement, quoting wordings from your verbal conversation regarding the kitchen.... 20k discount based on his accepted quotation... discount was not offered or taken, by way of any omission, part-omission or subsitution of the items contained and was a lump sum discount taken across all works contained in his quotation.
Providing your telling us the truth and not just looking for loop hole to get a more expensive kitchen (I`m not casting any shadow over you ability to tell the truth, this sort of disputes, are generally not as cut and dry as they seem). You would and should get your kitchen as agreed.
What form of contract have you signed up to? I am assuming the builders? You see he has had years to cover all loopholes and the like, watch out for a guy like this coming back with variations in cost due to P.C. Sums, Provisional Sum, Exculsion and all sorts of tricks.
If this went too court and the builder did not respond with in a reasonable amount of time, by the fact your letter remained unchallenged you would be in a very very strong position to win.
Hope this helps?