While I wanted to keep everything from the same source for convenience, it looks like RS was my next stop.

24VDC DPDT, contacts rated to 250VAC @ 7A / 125 VDC @ 7A (RS Online 699-6869)

As far as I can see, this relay should work in any of the proposed situations that involves relays powered from the 24VDC system. As DPDT it is straightforward to wire it as normally open/closed as required.

As far as mounting goes... I could hot glue the relay somewhere and fix wires directly to the contacts. This would mean glueing it upsidedown or on its side. For various reasons I don't really like this idea. There are are DIN sockets available (699-6881) that should be much easier to fix somewhere, and would give me screw terminals to work with. While it adds on another £3 per relay I think that it might save me a bit of worry.

Now I'm up to (in effect) £10 per relay. Hmm.

Let's look back at Irving2008's previous post with drawings. In the second drawing, Relay 1 is handling the AC current and needs to be suitably higher spec. Relay 2, however, is only handling 24V. A 24VDC DPDT relay rated to 220VDC @ 2A (619-3013) is a much more comfortable £1.78 + VAT. Seeing as the transformer isn't even supplying 2A in the first place, presumably that would be sufficient?

Since I started to write this, I note that Irving2008 has posted again. If "..the current passed [for the bleed resistor] is tiny (30mA) so the voltage rating of the contacts isn't an issue as very little arc will be generated", then would the 2A relay above also work in that situation?