Revenue operate under the terms of its "code of Practice."
Where errors are made, than can impose penalties and interest is compulsory at a whopping circa 12% per annum. The bad news is the penalties can be 100% of the tax at risk. They will view this differently and allow mitigation depending on a number of factors, including whether you make a "prompted disclosure" i.e correct after they have contacted you, re for example an audit or an "unprompted disclosure" i.e where you notify them prior to any contact which is the better position to be in.
The good news is, and hopefully it applies, is that where you discover errors in previous returns submitted and where the tax underpaid is less than £5,000 per bi-monthly period, you can make a "self-correction" of any errors on your next return, without notifying Revenue and suffering interest or penalties.
My advice would be to sort it out asap, in case Revenue contact you and the option to "self-correction" would be gone.
You can arrange to pay your VAT by DD. Revenue's ROS system is handy. You must ensure that the DD payments are at least 80% of your final years VAT to avoid an interest charge.