So my previous tenant didn't pay rent for a couple of months despite promising to. He's now claiming this is due to no hot water, albeit he paid rent for those months at the time and never mentioned this until I am now chasing in county court. Is he right ? I sorted the plumbing through a managing agent and they had problems getting access but it did take time.
As long as you have a paper trail of the repairs and can prove you were not being negligent of the repairs, I don't think he can use this as an excuse. There is an excellent website called landlordzone - the forum is excellent and I have used it for my tenant issues.
I believe there are very few reasons that he can with hold the rent legally. If it was the case he could have instructed the repairs and then either sent you the bill or with held the rent to that amount only.