@cathyk2002 - now that we no longer need to send in invoices other than computer/laptop I think the biggest expenses are heating in winter and broadband.
For broadband I work it out roughly as follows - how many days in the month, say 30. So divide monthly bill by 30. Then I multiple that my how many days I have worked, say 20. Then divide by 2 for half for worktime and half for non-work. So if your broadband bill is £30 per month, that's £1 per day, so £20 for working days, £10 for work (ie tax and £10 for pleasure usage). So approximately £120 per year to claim off tax as number of days per month vary etc.
Heating is a bit more tricky. Say you use gas for heating and your gas bill is £60 per month. For a 30 day month that is £2 per day. For 20 working days that is £40. However, you can only claim for heating the one room you are working in. So you have to count how many radiators you have in the house - say 5. Divide £40 by 5 = £8 per month, then it is half for working time and half for non-working time. So approximately £24 to claim per year if heating on for 6 months. People might argue a third due to sleep but I do not heat the house when I am sleeping.
If you use an electric heater which I do sometimes as well, I usually work it out as 1 unit of electricity per hour, so if your electric is 20p per unit and you use an electric heater for four hours that is 80p to claim. Don't forget to note if VAT is included in your unit price or extra.
Other people may work it out differently and maths was never my strong point but this works for me. Generally for running a laptop plus lights in winter I charge 1 unit of electricity a day (as the minimum can work out).
It is a real pain to work out, esp with all the changes in tariff prices and varying number of working days per month etc but being able to claim these expenses off tax is worth it in the long run, and you do not want to be cold when typing (though I found extra thick socks last year did help).