Hi Alex,
Good questions. This is a common situation, and you can usually fix it online. Here's how each point works:
1. Can I amend the CT600 online?
Yes. You don't need to write to HMRC. You file an amended CT600: a complete new version of the return, with every figure (corrected and unchanged) and the amendment indicator set. It replaces the original return in full, not just the lines that changed.
On WeFile, open your submitted filing and choose File Amendment. Your original figures are copied across, you correct the expenses, and the amended return is submitted to HMRC.
2. How long do I have to amend?
You can normally amend a CT600 up to 12 months after the filing deadline. The filing deadline is 12 months after the end of your accounting period, so in practice that's about 24 months from the end of the period.
After that window has closed, you can't send an amended return. If you've overpaid, you may still be able to make an overpayment relief claim in writing to HMRC within 4 years of the end of the accounting period.
3. Do I need to send revised accounts to Companies House?
Not if your statutory accounts haven't changed. If the mistake only affects the tax computation (for example, an adjustment between accounting profit and taxable profit), you only need to amend the CT600.
However, if the missing expenses mean the accounts themselves were wrong (the profit on the P&L was overstated), the accounts should be corrected too. In that case you'd file revised accounts with Companies House and send the corrected figures to HMRC.
When you start an amendment on WeFile, you're asked which of these applies: the tax return only, or the accounts as well.
4. Will HMRC refund the overpaid tax?
If you've already paid the original amount, HMRC will usually either refund the overpayment or set it against other corporation tax you owe. To speed things up:
show that a repayment is due on the amended return
include your company's bank details in the repayment section
check the HMRC online account for the company to make sure the repayment has been processed
If you haven't paid yet, just pay the lower amount shown on the amended return by the payment deadline (9 months and 1 day after the period end for most small companies).