I'm fitting a 240V roomstat to a W-B 24i Junior combi boiler.

I've run 3-core-and earth cable from the roomstat position all the way
to the boiler, where it emerges from the wall out of site behind the
boiler position.

However, I understand that you need to use heat-resistant 3C&E flex
within the boiler casing to connect to the terminals within, right? So
where is the correct place to interface the cable and the flex? The
only way I can imagine doing it 'correctly' is having a box sunk into
the wall below the boiler, adjacent to it's FCU, with a flex emerging
and leading to the boiler's roomstat terminals. But I've never ever
seen that done in practice... what's the solution? (Difficult for me to
do at this stage anyway, after the event...)

Thanks
David