everything for hmrc has to be fair and reasonable. If you can prove you've spent a portion of money for business purposes you can claim it. Such as having an electric heater running through the winter for the van, if you can work out how much electric it uses and the costs you'll be able to include that.
If your accountant says it's now £10 a month that'll be for a standard no questions asked expence. If you went to the trouble of working out how much the business costs you personally through household bills eg electric charging leisure batteries every day or the portion of the house / garage used to store business equipment you'll be able to apportion costs. As long as you can prove, if you had an inspection, you we're fair and didn't over charge you'll be fine