Under the Act as intended, the new employer cannot simply get rid of you after 90 days/1 year. They can only do so if the business environment changes substantially from when they took the jobs over and it is a highly debatable when that might happen. It is possible to make the case that for almost any business it is likely that SOME substantial change will happen in, say, three years, but of course it could be earlier or later.   That's one of the reasons why TUPE is so difficult. See for some background - paying particular attention to the first sentence.

What is required by the Act, of course, is not necessarily what happens in practice and because of the variability in the judgements, a new employer might find rulings to support whatever path they choose. Your union will be in the best position to give you guidance and support if it comes to it.