Presumably such an arrangement can be used to fill a bath should the
boiler
fail?

At 9.5kW, the bath will have gone cold before it has finished filling! You
can get 24kW inline heaters, which have the same output as a small combi,
but you'll need more than a single phase supply. They come in single phase
100A versions (which obviously need a phase to themselves) and 3 phase 33A
versions (which take a nibble out of each). Such a device could fill a bath.

Even if it's going to take half an hour to fill the bath that's better
than
having no hot water at all.

It would take about 45 minutes, if it stays hot enough.

Christian.