Yo, you not being thick I had the same issue when I got mine, its rather simple and you will kick yourself
Make Sure you have enabled the filter cut off to be automated on the virus control plug in -
1. Find the filter cutoff knob on the Virus control
2. Right Click on the filter cutoff knob and select the part you are enabling Eg. Part 1
3. If Filter cutoff is already there it is enabled for automation. If it is not you must replace an already enabled parameter for eg. Filter Saturation with the Filter cutoff by clicking on the parameter you wish to replace
4.Now that the filter cutoff is enabled you will hit the write option on the virus control (Cubase Option), hit play, and tweak your cutoff.
5. Now disable write and enable the read, if you are happy you can keep it. if you would like to clean it up or edit it you will find the automation under "VSTplugins" Folder (in the Cubase editing window) -> "Virus TI" and if u click the automation there should be a Filter1 cutoff* select that and you will be able to view your automation
Hope this helps