I am looking into this too
If you want your water to heat up at different times from the central heating coming on..then you need a 2 channel one. Otherwise each time you heat the water, your heating will come on unless you swutch off the central heating function.
This is the best i have come accross. Look at their own website to see which pack you need..
Honeywell Sundial RF2 Pack 2 Wireless Timeswitch | Time Switches | NoLinkingToThis