1. Which Honeywell stat have you bought? - It is the DT200
2 Do you have a combi boiler or a Hot water cylinder with one or two motorized valves? - I have a Hot water Cylinder, how do I check the number of valves?
3Where do the other end of the stat wires end up? - No idea?
Can you see your pump? If so, the pipe will divide in two after the pump. One branch goes to the HW cylinder, the other to the rads. There will either be one valve (left pic) at the junction, or two valves (right pic), one in each branch.
If you have the valve on the left, it will have a multicore cable attached. Find where the white core is connected. If the yellow wire from the stat is connected to the same point it is the switched live.
If you have two valves, as per right pic, find the one which goes to the rad. It will have a multicore cable. Trace the brown core. the Yellow wire from the stat should connect to the same point.
Click this link for
DT200 Instructions
You will see that the DT200 does not need a Neutral so this wire can be wrapped in insulationg tape and hidden behind the stat.
That leaves two wires: Red connects to terminal A; Yellow to terminal B.[/url]