One of the obligations on being in business is keeping records for tax purposes.
Earlier this year HMRC published a helpful factsheet on the minimum requirements:
www.hmrc.gov.uk/factsheet/record-keeping.pdf
Its worth reading this and making sure your business is compliant – if its not, you would have little defence against HMRC penalties in the event there was a tax enquiry and HMRC claimed you were not keeping adequate records.