Is this normal procedure?

2 weeks ago the timing belt went on my brother's fiesta. He took it to my normal garage to get diagnosed. They took it apart and changed several things, like the belt, water pump and rocker head and said this will cost £600. Then they discovered it was misfiring and stuff so took it apart and said there was their damage, so it will now cost £900, and then today they have said it will need a new engine cause the pistons are cracked and begin is cracked.

So my questions are, should they not have discovered all this damage when they first took it apart to change the belt? and should I have to pay them the £600? because now I what t take to a different garage.

Personally I don't think I should have to pay anything because they misdiagnosed it when they first took it apart.

I appreciate any advice.