Realy annoying intermittent central heating problem.

Can any one help (i hope so )
parts...ideal classic rs50 gas boiler, cadet(s) pump , sunvic select 207 programmer, sunvic tlx7501 digital room thermostat , sunvic clamp on tank thermostat, sunvic 3 port motorised valve . As far as im aware a normal pumped system with 2 water tanks in loft and a hot water clylinder in airing cuboard.
Ok, here goes.My room stat is set at 22 degrees.In the morning when the timer comes on the central heating wont start (even though the room stat is reading for example 17 degrees).The hot water is fine .
So , some times i can go to the thermostat and turn down the set point to below whatever the room temperature is ..then back up to 22 degrees and i here the boiler fire up (and pump starts) and bingo it works .
Other times after trying that and it will not work ...if i go to clamp on thermostat on tank (normally set about 65-70 degrees) and turn that up about 10 degrees (80) bingo it fires up ok .
Its never consitant , during the day if it reaches its 22 degrees , it shuts down and then when temp drops starts up again .This has been going on for about 2 months now and nothing has been replaced (except a new thermostat to try ) or altered . I fitted all the sunvic parts myself about 9 months ago and everything has been working fine .
The last 2 weeks iv noticed it never fires up first thing in the morning , and will only seem to work if i turn up the clamp on thermostat on tank ..it is now set at about 80-85 degrees which seems to hot as you cannot touch the tank..
I don't mind replacing any parts its just Knowing which to replace ..or if any body has any ideas you would be a life saver..sorry for long post but i thought it better to explain more than not enough ..

So all your bits work, but not all the time. Have you checked all connections - physically and against the instructions?

Yes iv checked all conections...behind the timer , the thermostat and the connection box inside the airing cuboard ...nothing seams adrift and all teminals are tight

I think you need first to prove that the cylinder stat is controlling the HW satisfactorily (with CH off).The reason I say this is because the stat supplies power direct to boiler and does not rely on the 3 port valve. Secondly you need to prove the room stat is functioning correctly, this time with HW off. The stat powers up the 3 port valve through the white wire and should move the valve to the CH position (unless it is already in that position) and also supplies power to the boiler through the orange wire. A 3 port valve can develop problems such as 'a stiff spindle' or a 'faulty motor' or 'a faulty microswitch'
Mine turned out to be a faulty microswitch which meant I only had CH when the demand was for both HW and CH. I could not have CH only because the valve did not supply the power to the boiler even though the valve moved to the correct positon.

Is the cylinder stat a good thermal contact with the cyl?
Haven't looked room stat up but does it need a neutral wire and if so has it got one ?

@chris r
the cylinder stat is attatched ok and appeares to be working fine ..the room stat does not need a neutral(although there is one there )

at the moment the room stat says 20 degrees...the room stat is set on 22 degrees...i have a litle flame icon to say the heating is called for...yet it does not come on...if i go to my cylinder stat and turn it to 75 degrees (it was set to 70 degrees) it will start up the boiler ...start the pump and the 3 port motorised valve will activate ?very strange

Suspect sticky 3 port. Assuming its a spring-return type (what's the number?) it would have to go to the end for CH and operate the microswitch there.

instructions say SDMV SERIES 3 PORT MOTORISED VALVES AND SDM1901 ACTUATORS..on valve itself just says sdm 1901

manufacterer might help pleese

Possibly sunvic - see first post.

yes it is sunvic as i explained at the beginning of this post .

Iv not got a problem changing the 3 way port if thats what everbody thinks ...what i can't get my head around is the fact that if i turn up the tank stat it fires up and works fine!!

As I mentioned before the cylinder stat relates to HW and is not dependent on the 3 port valve to supply power to the boiler. You still need to isolate the problem. Ignore the CH, turn it off and try HW only. Will the boiler respond to the cylinder stat without manual interference. Will the boiler start if you run off some HW and will it stop when set temperture is reached. If there is a problem at this stage then I would consider the stat to faulty.
If you get passed the 1st stage then ignore the HW and turn it off, and try CH only. If it does not start the boiler, it could be faulty room stat, or faulty valve (splindle? motor? or microswitch?) or wiring connection.
I think you'll get more confused if you have both HW and CH on together.

@mandate
Cheers i will try you suggestions ...although i have tryed a new room stat only a couple of days ago so i cant see it being that ..will let you know ..cheers

@mandate
checked the tank stat ..all working ok ,so all thats left is the 3 way port (iv already tryed a new room stat).
Changed 3 way port and all working ok now ..Thanks for all your help...