That would't be hard would it. They were involved in the proposed move to the Butts, had reportedly been shown round and were happy with it moving forward. Then there is public record of the council moving to block it. Anderson publicly stated staying at the Ricoh was an option and negotiations were taking place. Wasps called a halt to talks. Again that is publicly known.
Who knows what else they have in their pocket. Kieran Crowley has categorically stated, since leaving the club, that he has seen documents showing the council blocking two other sites.
And the point is SISU wouldn't necessarily have to prove it, if the FL can't afford to defend a case they won't want it going to court in the first place.