Realise interlinked alarms require some work.
You need to get the link from ground-floor ceiling to
hall-ceiling - which may involve major redecoration.


If you are going to fit interlinked smoke alarms, you
may as well use mains powered w/battery backup.
EI 141 are cheap - just £12-14 on Ebay, long life.

Wiring...
Wire off a local lighting circuit whose loss would be
obvious - such as the hall lighting circuit. Use 3C+E
for power-&-interlink, NOT the E conductor of 2C+E
(fails inspection & no-one expects E to be live).
Ensure L+N polarity is correct incidentally.


An alternative is to use Wireless Interlinked Alarms,
EI have a range of these - wireless via the bases.
Obviously this puts price up quite considerably.