CH problem

I have a Potterton Flamingo 40 boiler downstairs, a hot water storage tank upstairs and two open tanks in loft.
Hot water works fine but CH does not work at all. Boiler fires up for CH but cuts out after a couple of minutes.
Checked tanks in loft and they're ok (2/3 full)
Thermostat (only one in house, on wall of living room) is set high enough and I think it's working as I roll dial up and down past room temperature I hear it 'click'
Thought problem might be pump as I heard intermittent whirring. Replaced with like for like (Myson CP53). seemed to work at first but after an hour same problem returned. Follwed manufacturers' advice on turning it manually but same problem on restart. Pump now runs but rhythm still isn't constant. ie seems to speed up then slow down. also a water 'rushing' noise every minute or so. Problem same on all speed settings, sometimes accompanied by occasional whistling noise.
All rads have water (bled them all).
Boiler sometimes fires up when adjustment to it's dial (1-4)made but same problem returns.
Could problem be sludge in system?
If so, what should I do to solve it?
I have a SUNVIC controller if that's relevant.
Any advice would be greatly appreciated.
Thanks,
Graham

sounds like the port valve to me

You need to check that there is adequate water in the system - try venting one of the top radiators (with pump off). If air comes out, continue venting till water shows. If nothing happens, you may have a blocked feed and expansion pipe (from the smaller tank). Post your results here.

Thank you both for such a rapid (I'm talking minutes!!) response.
Removed the port valve controller, turned valve manually and CH came on!
working fine at the moment as timer dictates but will this mean hot water will run out at some point and I'll have to turn valve back to heat the storage tank?
I will check power to the controller before replacing it. Should there be 240v across it at all times?
Will also check there is adequate water in system with pump off as advised to avoid any other future problems.
Again, thank you guys. Wife is warmer and happier and I've avoided becoming any more poorer.