Heating coming on with water.

I have a Stelrad Ideal Ilan boiler with a Randall 702 programmer.
If I just want the water to come on I can time it and have it so that the heating is off but - the heating is coming on with the water.
If I manually turn the water on for an hour - the heating still comes on.
I have turned the thermostat (on the staircase) down to zero but this doesn't stop it. I have the water thermostat (attached to the lagged tank) set at 50, if i turn this right down, it stops. It hasn't always been like this but it's been a bit "iffy".
How can I stop the heating coming on when I just want the water

Help much appreciated.

It sounds as if your motorised valve may have failed!

Tony

I dismantled the valve and found that there's a little black ball type thing that shuts the water off, this had come off the stem and was just floating free.
I've put it back on the arm and it's working o.k now.