Quick update - relay and Raspberry Pi all wired up as instructed.........and.......(drum roll please)........it's working!!! Hooray
Huge thanks for all your help Steve - I would have never of done it without you!! I even got time in-between waiting for parts to arrive, to convert my existing wired alarm system to be a smart one. The alarm can be armed/disarmed by an app, voice control and even knows when there's no-one at home, so sets automatically. Anyhow - that's a whole different thread!!
Voice control added for the hot water and heating - now just need to work on implementing a heating schedule.
Phase 2 - incorporate temperature sensors. The old thermostat is still in use, but would value people's opinions on what's possible here. Nice to be able to measure the temperature in different zones - e.g upstairs & downstairs - although I realise, with my current setup, I can't control the heating in two separate zones.