Yes I can recall it happening before. It is not right and is in my opinion mortgage fraud.
Once they have your money it could go on for many months.
I would add that if Gleeson are incapable of planning for the electricity to be connected at the time of occupation, I wouldn't trust them an inch to build a new home correctly.
Please have your home professional inspected and snagged before legal completion.
My advice would be NOT TO LEGALLY COMPLETE without mains electricity.
Generators can be and are noisy, there will be cables everywhere and the diesel can run out, perhaps at the weekend!
If you ignore my good advice and legally complete anyway, have your solicitor get a firm date of when the electricity will be connected by and I suggest a £300 a day penalty be written in the contract for every date beyond this date it remains unconnected.
If you are forced to legally complete, I suggest you inform your lender that the home will not have mains electricity on the day and I expect them to withhold the mortgage monies as your new home will be virtually worthless without mains electricity.