I have many friends who work for TCX, and have done now for the 3rd season in a row.
They havent been kept on any winter, as has none of the other courses.
They never keep on NE through the winter, but invite them back the following March/April.
After you have done a few seasons they are more likely to extend the contract.
Hope this helps.
AEU, FJE and sometimes FCA are the only charters that consider their NE crew for permenant contracts.