You might want to post up a copy of what you have.
If it is actually a fine (and it could well be since this is one thing Advantis do for the DVLA) then you have been prosecuted in the magistrates court.
There is a sticky about how to get a prosecution you were unaware of set aside.
However the reason you didn't get a reminder - for which there is precisely no obligation - or any of the documents before prosecution is, bluntly, of your own making.
Having got the prosecution set aside you can attempt to convince the DVLA to offer an out of court settlement (far cheaper but very unlikely).
The almost certain result is that they will simply prosecute you again, almost certainly resulting in the same fine, back tax, costs and victim surcharge.
However it should at least mean enforcement costs are saved which may be worthwhile.
