When an impairment is posted to an asset the depreciation is not adjusting to new value until following year. For example
Asset 10k with useful life of 10 years will depreciate at 1k per year
Impairment posted of 5k so value is now 5k
Depreciation should be £500 a year in year of impairment and following years, at present it is charging 1k in year of impairment and 500k in following years.