Official error: the applicable period: unreasonable delay
ESIM2300 explains that a claim for statutory interest in respect of official error must be made within the 4 year time limit set out in law.
ESIM4500 sets out the sorts of delays to be ignored when determining the applicable period; one of those is unreasonable delay by the taxpayer in submitting the principal claim on which interest is claimed.
Although we want taxpayers to make claims promptly we cannot lay down any hard and fast rules for the submission of claims other than the legal 4 year time limit. This means we cannot insist on submission of a claim for the principal amount within say six months of discovery of the official error because this would have no legal basis. It would therefore be rare to deduct a period for unreasonable delay in submitting a claim when calculating the applicable period.
What is actually considered as unreasonable is to some extent subjective and depends on the individual circumstances of a particular claim. However where appropriate it may be possible to take account of any period of delay by the claimant in providing further information see ESIM4700.