TUPE - a phrase that makes you shudder everytime its mentioned.
Transfer of Undertakings, Protection of Employment = TUPE.
In a nutshell, one company can sell its business to another including all staff, as an employee you take with you all your benefits. However, the new company can and normally do nip away at the benefits if they are better than the ones their employees are on.
One big sticking point, pensions are not normally part of TUPE, new employers only have to offer you a broadly comparable pension scheme. Trust me broadly means broadly.
Another point is, if you are lucky enough to get promoted, your promotion would/could be subject to you signing a new contract for the position under the new companies terms and conditions.
It is a mine field.