Don't bother. With all due respect to Greekman (and I think he's even said that it was totally his own fault), if you drive within your limits then you won't ever have an issue due to anything you've done, and if the problem comes from around you (i.e. people driving like tits) then either stay well clear or, even better, report them to the TDO on the day who should sort it. If they don't, then at least you know not to spend your money there again.
That said, if you can get it built into your main policy when you take it out then go for it. I'm with Richard Egger mainly because they gave a great price and don't care at all about mods, but having trackday cover added in is a nice bonus. As Darren says though, the excess is going to be at least 10% of the cost of the vehicle, and you'd have to be doing something really stupid to cause that much damage to your car in the first place. For all the damage that Greekman did, it's not £2000 worth there so even if he'd had cover (he may have done, I don't know) it still wouldn't have helped him. I didn't even bother with it when I had the VXR220 despite each clam costing £2500 to replace as it simply wasn't cost-effective.