If you refuse a fixed penalty the prosecution has six months from the date of the alleged offence to bring the matter to court. If you are summonsed to appear you can plead not guilty if you wish, though from your description of events it seems unlikely to be a successful plea and you will probably be found guilty at trial. The documentary matter you mention will make no material difference to the outcome.
If the matter is dealt with in court sentencing guidelines suggest a fine of half a week�s net income, plus 3 penalty points. (Like all offences which carry an endorsement the magistrates can disqualify you for this offence alone, though this is most unlikely). The fine will be reduced by a third if you plead guilty. You will also be liable for a contribution towards prosecution costs (about �45 if you plead guilty, about �350 if the matter goes to trial). In addition you will have to pay a �15 Victim Surcharge.
As has been correctly pointed out, it is the date of the offences which count for �totting up� purposes. If the dates of the earliest and latest offences which led to you accumulating 12 points are within 3 years then you will face an automatic disqualification for a minimum of six months. You can plead that �exceptional hardship� will follow if you are so disqualified. This is not an easy argument to win.
In short, if you go to court and plead guilty it will cost you at lead a couple of hundred pounds. If you plead not guilty and are eventually convicted, it will cost you a lot more. Either way you will still get three points and so become eligible for a �totting� ban.