This post might not be an use to you now ,but for future reference you might want to take this on board for any other builds.
There are a few published standard forms of building contracts. The most common is JCT building contract for homeowners and occupiers.This form should cover you from any minor to major **** ups by the contractor.This form allows you to with hold 5 percent of the aggreed contract price for up to 3 months ( this is to ensure the contractor is motivated to return + remedy any defects in there works.0 you can also aggree on paying the contractor a % of cost to the % of works done. You can get these forms from any building book shop or building organisations.
For a more insight to what i have posted you can purchase or hire this book from any libary.
"LAW FOR HOME IMPROVERS AND SELF BUILDERS" by Guy Elyahou.
hope this helps you or any one else here.