What can replace the old and very basic Drayton Digistat 2. Among other models, two that are easy direct replacements would be the Honeywell CM907 and the Honeywell T4 (Both battery powered models, not mains).
Alternatively, you can also replace the Digistat 2 with much more up-to-date models, such as certain models of HIVE (depending on your system) and the Drayton Digistat +3. Just be sure to get the wiring / terminals right.
Replacing with a HIVE can be a little bit more complicated due to some systems, such as combi boilers, needing some adjustments electrically to be fully compatible. So some won’t be fully operational from a direct programmer swap / rewire.