He beleives he was justified in not asking for me to do the blockage clearing because A) there was no guarantee of success, B) no guarantee when I could start ( there was, I told him within 2 days ) and C) that a matching tiles may not of been found to replace the ones I would remove - even though we did the job only 2 months before!!
If he's stated in writing that he didn't ask you to rectify the problem you should be off the hook surely?
As stated above he has to give you the opportunity to remedy the situation.