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Ask the architect about using the RIBA contract. In my opinion it is more in favour of the home owner, the JCT contract is in favour of the builder. RIBA is really mainly in favour of the architect, who is god, but I would expect you and the architect to be more often in agreement than you and the builder. There isn’t a “homeowner” organisation that has a contract that is actually in favour of the client as far as I know.
I would not use the JCT contract.
This is definitely the advantage of going for standardised contracts - everyone on the same page.
The only thing about holding back such small % is that it may be built into the price.