We probably shouldn't ask you to show your working here, but
- remember to count each trip to London as one journey, and each trip back as another one (so two trips a day)
- think hard about how often you have done this, starting from how many days a week you go into London. You might think that (say) you go into London three times a week and only ever fare dodge on the first time each week: think hard about whether that's really true. It's very easy to underestimate how often you do something. So be brutally honest with yourself.
- now look at the number you have come up with. Is it something like 899? Or is it maybe something like 89 or 99? I'm thinking there's a possibility that someone's finger may have slipped when they put the number of journeys into the spreadsheet with the practical result that they are trying to charge you for (very roughly) ten times as many trips as they should be doing. Of course, that will get you nowhere if you reckon you have fare dodged 25 times (or 250 times i.e. some number which is wildly different from both what the railway have said, and from an easy typo of what they have said).
In the circumstances, I think it makes sense to ask Southeastern for their figures sooner rather than later. If there's been some sort of miscalculation that may become obvious once they send it through, and you may be able to wrap the whole thing up once that's sorted out.