Had the Part P scheme been properly enforced and all trades/public informed to it's importance then you and many other would not find themselves in this situation.
I wouldn't feel too badly about making this mistake. Guaranteed you don't make it again.
I would certainly make your mate sweat some though, he has broken the law if he is unable to certificate the installation so put the pressure on so that he realises the importance too
Do you know if your mate has tested the work that has been done so far?
In my opinion your major worry would be insurance.
As others have said, very few members of the public are aware of Part P and the requirements - but you can bet a years earnings that the 'Insurance Companies' are aware.
If, God Forbid, something went wrong and you had to submit a claim for damages, I'm pretty sure the first thing the insurance company would say is 'Where is the 'Electrical Installation Certificate' and 'Completion Certificate' for this installation?'
And producing a PIR might not be good enough - they look for any loop hole not to pay, remember.
Best course of action is get down to your local 'Building Control' and tell them what has happened and do what they advise - regardless of the cost.
Possibly even consult your 'Insurance Company' and get confirmation in writing that your proposed plan to put this right is acceptable.
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