Late to the party, but…
…elec bill was £57/month but since Mums not there to turn off the lights the bill has risen to £100/month.
I agree that this is at least twice what your stepdad should be paying, since he doesn't use electricity for heating. It's always a good idea to try and get a cheaper deal, but it would be even better to track down why the bills have gone up so much.
If your billing system is like mine, the power company sets a semi-fixed monthly bill that will only vary when the company changes its tariff or your stepdad changes the amount of electricity he uses over a period of several months. I think he was indeed using the extra power, they started to bill him for it, and now they haven't yet caught up with the fact that thanks to the LED lamps he's now using less.
My wife and I use about 3000 kWh a year, including all cooking, and that costs us about £46 a month. 3000 kWh a year is equivalent to a load of about 350 W running 24/7. So if your stepdad had just half a dozen old-style bulbs – 60W or 100W each – switched on day and night that could certainly double his consumption and push his bill up to £100 a month.
You've replaced most of the bulbs with LEDs, so that will have cut the "waste" consumption by about 85%. But depending on how often the meter is read, the power company's billing system may not have caught up.
I suggest you try to get some actual meter readings (not estimated ones) from his bills, or just read the meter yourself. With the LEDs fitted you should see perhaps 8 kWh per day, and the same when your mum was alive. In between those times, you'd expect to see almost twice that, so say 15 kWh per day.
Once you're sure that he's back to using, say, 8 kWh per day, call the company and ask them to reduce the monthly payment. They might insist on waiting for the next meter reading, but it has to be worth a try.
@Slugsta suggested a smart meter, and I can certainly recommend that. My dad's just had one fitted, and I can log on to a web dashboard to see not only how much electricity he uses per day but also when he uses it, in half-hour slots. At home we have a
geo Minim+ energy monitor that also works very well. If your stepdad is still using more power than seems reasonable, monitoring hourly usage can help you track it down.
Since it's been a couple of months since you posted, I hope perhaps the situation has fixed itself.