UK umbrella offering 80% retention for contract in Belgium

[gardener]

folowup on Optimum Pay Group

Hi Contractors,

Got a sugestion to go through an umbrella "The Optimum Pay Group" for a contract in Belgium. They offer 80% retention. Too good to be legal, looks like. The representative from umbrella said it's because of 183 day rule...

How does that sound to you? Anybody delt with them before?

It's not a loan, some scheme with high personal allowance I think.
If I am employed by them, can HMRC hold me liable for taxes they didn't pay though? Looks like this is the key question. They as an employer have full responsibility to pay all the taxes for me, don't they?

To follow up: Optimum Pay Group is a guy - a small time crook with worms in his brain. Stay away.