Hetty58 the fact that your combination microwave has metal shelves when using on combi programmes, still does not mean that you can use metal in it when using a microwave programme.
The different programmes cook very differently. So in combi mode - Yes you can use any metal including roasting dishes, and cake dishes, etc. But when in microwave mode, all metal is an absolute NO, NO.
Best way to prevent anything from overflowing is to use large containers.
And, instead of using clingfilm (plastic), there are very many types, and sizes of perpetual covers for jugs, dishes, plates, etc. which can all beused in microwaves.