Who to pursue for unpaid time when working via an umbrella?

[SueEllen]

If you feel that strongly that the client is a cowboy outfit then name and shame them. That's the best way to warn other contractors. Do it here, do it on their twitter and Facebook feeds and do it on other job sites. DO it anonymously though. If you are the only person with a problem with them then you'll be ignored if not then others will respond too and they will quickly realise they are getting a bad reputation and change their ways. Be careful to stick to pure facts though!

You don't even have go that far.

One of my ex-employers now has a recruitment issue both permie, contractor and with off-shore resources due to their previous conduct. I've had agents ring me up, tell me this and all I have to say is they do have issues. I've had discussions with off-shore people who were badly treated by them and just let them know they treat all their workforce like crap.

I also tell agents why I won't work for another client due to trying not to pay an invoice and advise them to be careful.