Our central heating thermostat went kaput. It is a simple Honeywell
T40 and it is a "two-wire" item.

I couldn't get a direct replacement so chose a Siemens RAA20 which is
also a "two-wire" item.

So far, so good. But on opening up the broken Honeywell, while the
wiring diagram inside the cover shows two incoming wires, L and N, the
wire from the controller has *four* cores and they are all connected.

Green/yellow goes to earth terminal (E). Blue goes to neutral
terminal (2). Yellow goes to terminal 3. Red goes to live terminal 1.

So I thought to myself, how can this be a "two-wire" item?

Just to complicate matters, the replacement Siemens RAA20 is also
described as a "two-wire" item, but it has *three* terminals. L is
clearly live, but there are two other terminals Y1 and Y2.

Now I am totally confused. Can anyone please tell me which of the
four original wires (red, blue, yellow/green and yellow) goes to which
of the three terminals of the "two-wire" Siemens RAA20?

Please help! It's getting cold in here!