Agency withholding pay due to "investigation"?

ETS:
I’ve submitted an “Early concilliation notice” through the acas; problem is - I have no actual proof for work done, no contract for work either.

Yes you do. As long as you never deleted the texts from the agency you have proof of a contract sat on your phone. The proof you did the work is is sat on your digicard and will be there for a few months at least. If you can get to another truck or find a friendly transport company or TNUK member to do a print out of those days then you’re good to go.

Just going back to the text messages from the agency, always get them to confirm everything by text and don’t delete any of them until they’re several months old because you’ll never know when you’ll need proof of what they said.