I've completed phase 1 of my home installation.
This is what I have in my existing system:-
Ideal logic 24 boiler
2 x heating zones controlled by 2 x stats wired to 2 x 2 port valves.
Unvented Megaflow cylinder.
Hot water controlled by a 2 port valve.
TRV's on all Rads except 1 in each heat zone.

Evohome kit comprises of:-
2 x BDR 91 relays replacing the thermostat controlling the heating zones.
1 x BDR 91 relay replacing the hot water timer controlling the hot water 2 port valve.
Hot water insertion probe connected to Megaflow cylinder.
Evohome controller set to Ground floor as temp control for zone 1
DT92 stat on 1st Floor as temp control for zone 2 (1st and 2nd floor).

All seems to be working ok so far.
I've created this post for reference so I can refer back when I pose other questions and I will update as the system grows.