Not that I think it matters but how long was the break in employment in 2013?

But since the document doesn't say that it doesn't continue during any termination and the fact that you accepted the secondment that referenced the document I would bet that it has a pretty good chance of standing up in pretty much any vinue in the US.

I deal with several companies that are temp firms. They often have an agreement that is signed during one period of employment and if they are using the same version of the document they don't have them sign another. The have held up in UI, Work Comp and even EEOC cases.