The '7 year clock' only starts running when an 'outright' gift is made. Full liability for IHT remains unchanged when a 'gift with reservation' is made.
So even if you could prove that your father gifted the house to you many years ago (which I doubt), since he still retained the legal right to sell the property for his own benefit (even if he had absolutely no intention of doing so) it would have been a 'gift with reservation' and it wouldn't matter if he died more than 50 years later, full IHT would still be due.
Similarly if your father now gifts you the house formally (by changing the title register) but retains any type of interest in the property (e.g. by living with you without paying a full commercial rent) it's still a 'gift with reservation' and the '7 year clock' doesn't start running.
It's worth remembering though that IHT is only payable on the value of a property above £325,000 (at the current threshold). Further, the 7-year rule doesn't represent a dramatic cut off point. It's a sliding scale, so you'd pay less if your father died (say) 6 years after making an outright gift than if he did so only one year on.
http://www.hmrc.gov.uk/inheritancetax/intro/basics.htm