Be wary. When you take your car in for a service you pay up and then take it away, if you don`t or can`t pay then you don`t get your car back. This is where the construction industry falls down. Trouble is so say "property developers" think they are doing tradesmen a favour all the time, but forget they can`t get the job done without us. Bugger the retention, he will only hope you forget about it anyway, try getting it back after a year, especially if he`s gone to the wall, its your money not his. Stick to your guns, he won`t do you any favours. If he really wants you to do the work then he will agree to your terms and conditions.