HMRC requires you to keep all your business records for VAT purposes for at least 6 years.
In normal cases, the HMRC tax investigation time limit is 4 years, in which they can go back to claim money from taxpayers.
The maximum period for HMRC to raise an assessment is four years from the end of the VAT return period to which the assessment relates. However this period only increases (to 20 years) in cases of fraud or dishonesty – clearly not the case here!
Applying the four year time limits to this assessment, it is clear that the period covered extends beyond the four years allowed. Therefore the assessment are out of time and are not valid.
In light of the above, you should write to HMRC challenging the assessment. This can be done as part of a reconsideration request; OR you can choose the option of appealing the assessment to the First Tier Tax Tribunal.