heating and hot water won't work independently

The title says it all. I cannot control the hot water system (boiler and cylinder type) without the central heating coming on (and vice versa). I assume this is due to a seized (due to sludge) electric splitter valve (sorry but I don't know the correct term). If I wanted to replace it would I need to drain the whole system, just the cylinder or something else?

When I replace it I plan to install valves on its output side (one to the CH and another to the HW). Is this ok? I'm just thinking about potential problems: if after fitting, both new valves were closed and the HW and CH were switched on, what would happen?

Thanks.

I think your splitter valve could be a 3 port mid position valve.
If it is, then you should be able to remove the 'actuator head' from the spindle. You can then try to see if the spindle turns easily. You can also test the head whilst is off the spindle. To replace the valve body you will need to drain off (perhaps not the whole system, but down to the valve level), but if you're just fitting a replacement head then no draining needed.
Have a look on the honeywell site, it covers the 'Y' system with 3 port mid position valve and the one with two diverter valves. I think it's the 'W' Plan.