No. It has been stated previously and not just by myself that driving with red diesel (Gasoil) in your tanks is not an outright illegal activity.
A vehicle can quite legally fill up with red and then pay the outstanding duty AND vat at your local customs house. Admitedly not the most convenient of operations, but certainly, during the fuel crisis several years ago there were more than one driver that I know of, out of necessity who did just that. Understandably HMRC aren't too keen, I imagine it's a consideral faff because of the paperwork and it isn't something that's often requested.
A small point but I did qualify my statement by saying HMRC would have dificulty dipping AND taking action...but I agree and am well aware of HMRC's abilities;)