When real time Capital Gains Tax (CGT) has been paid and entered into CGT10, the amount of CGT paid appears on the tax calculation summary (SA302) and is deducted from total tax due. If there is subsequently no gain to report , the return will fail filing return with a CAL1 error, the difference being the amount of CGT deducted.
HMRC systems will currently not accept a refund of CGT when there is no gain reported. Consequently a Special Case (50) has been documented and provides detail on how to workaround the system failing.
To enable any CGT overpayment to be offset against any Income Tax due, an amount of -£1.00 should be entered in box CGT51. A note should be included in box CGT54 (Any other information) on page CG4) to advise that the workaround has been used.