Huge Electric Bills

Gerry1

The flashing red LED on the electricity meter on the wall is your friend.  It'll be marked 1000 Imp/kWh or suchlike meaning that 1000 flashes (impulses) mean you've burned through 1 kWh costing 15p or whatever.

If you count how many times it flashes per minute it's very easy to do the sums.

If you turn everything off and then turn on circuits / appliances one at a time you'll soon get an idea of those that cost the most.

dunstonh

On submitting a meter reading, I found that we had used around £120 of electricity in the space of a month. 

Do you give a meter reading each month or was this the first one for a while?

If you don't do it monthly, then the meter reading you have just could be correcting previous estimates and result in this months statement jumping up in cost.

We don't have a smart meter as we don't want to reset our meter as the clock is out in our favour.   However, we do use the following:
geo Minim+ Energy Monitor - Self-Installed with LED Sensor for modern electricity meters: Amazon.co.uk: Garden & Outdoors

That was instrumental in us halving our electric costs by allowing us to identify what was causing heavy electrical use.

7Phil

Something is sucking the juice. You said you do a correct meter read each month, so double check the readings/kwh you use each month previously and compare. If you use a consistent kwh reading each month (let's say 200 every month as example) but your £ cost has bounced up then there has been a tariff change. That can happen if you ended your contract and defaulted to the "standard tariff", as some suppliers do sneakily.
If that is the case 1) switch to a current tariff and 2) complain and ask for a refund of the extra costs you incurred - no harm done by complaining.

If you instead find that your kwh readings have jumped up then there will be a culprit within your home.
You mentioned your son moved out and therefore correct to expect a reduction in elec usage especially if he was a nightowl!

Phones and tablets are negligible but laptops and telly's can use a surprising amount of elec (even LED ones). You said you only charge laptop 1x per week so unlikely an issue there... Either case, neither of these will bump your bill up a massive amount unless they are faulty.

Extra fridge/freezer is a big issue especially if either is an older model, going EOL has developed an issue with the door seal. You may be answering your own question by saying one clunks and rattles and stays on a lot.
Do an elimination test by leaving it off power for a day or two if you can. Check your meter reads to the decimal point each day to compare.

theoretica

If you look back over your monthly readings do they show a gradual increase or a sharper change where your usage increased?  Did anything in your home change about the time the readings did?  Anything new (and electrically powered) bought?  Something seeming not to run right?