If by Nasty party you mean lefty hating then you have a good point. Most likely the party would see Camerons softly softly approach as having not worked. Whether or not it will leap totally to the right is another matter. Personally I would prefer a move more to the right, although a Tory I am not a cameron fan.
Of course the same could be said of the labour party. Polls have shown without doubt (whether they win or not through a flawed voting system) that they are out of favour. Will they lurch more to the Left and hard line Unionism ?
If we get a hung parliament it would be unlikely to last a full term, so I reckon you could be right, but I think both parties will re align so could get interesting again.