hot water/no heating-diverter valve?

I have a Gloworm spacesaver kfb50, Grundfos 15/50 pump, Sunvic SD 2701 three port actuator.
The hot water works fine but the heating won't work at all.
I've put the thermostat right up and switched the heating on either with the hot water or on it's own but cannot get any heating.
The pump is working and comes on when the boiler fires up but the diverter valve does not seem to allow any hot water into the central heating. If I move the lever on the side of the diverter I can feel the pipe on the central heating side get hot but it automatically moves back once I let go.
So my questions are,
1 :Is it probably a broken valve motor or could the programmer/thermostat just not be "telling" it to open?
2: Is it acceptable to take the valve motor off and manually open the valve so allowing some heating for a period of time? The valve does move quite easily if I lift the motor off.
P.S The system has been intermittantly creating a droning noise of late ( sounds like a Lancaster bomber circling the house) and the central heating has been going on and off without reason. Thanks for any suggestions.

Your symptoms are indicative of a broken sync. motor in the valve. To check measure the voltage at the motor connections inside the valve head when calling for CH.
Motors are easy to get and fit, just the usual electrical safety precautions which, if you're competant will not need explaining

thanks for the speedy response. I should be able to sort it if it's just the motor.

Hi mate just read ya thread. Some sunvic valves wont let u change the syhncron motor & u have to by all the head. Did go to a sunvic last year that was burnt out .when i told the customer he said it cant be its only been on 18 months & the original lasted 10 yrs. the flat sided spindle showed signs of weepin so WD40 an 50 revs . free as a bird.should be able to turn it with ya fingers easily.

There is some corrosion/scale on the plate the motor sits on but I can move the spindle with my fingers with out to much effort but I wouldn't say it moves real easily.

You CAN change the motor on a SD2701 (you can't on the 5601/5651) but if the body is starting to leak, which they do with a vengeance, it would be better to change the whole valve.
If it's impossible to get the valve out reasonably easily because of silly plumbing - all too common - you can get a repair kit which you can fit into the existing valve body. It's a lot easier to buy the same valve again though (body is 2702 if I remember...) and pinch the moving bits from the centre.

got the motor, £40 successfully fitted it, took around half an hour from start to finish after squirting some wd40 around and turning the valve a few times. Having taken a closer look there is sign of quite a bit of seepage over time so i will change the valve shortly but I guess I'll have to drain the system for that as there are no isolation valves adjacent to it.
Thanks for the help