minimal heat from radiators and hot water system?

Since turning the heating back on our hot water has cooled down to minimal heat and the radiators do not get hot at all even when the stat is turned up full, boiler was checked over when heating was put back on as it wouldnt light shortly afterwards, this was the fan and now it lights no problem, heat was a problem before this but as weather was still ok we did not realise until after the engineer had left so couldn't ask him! boiler is fine and the pump feels as though it is working as it does slow down when turned to a lower speed and hums quietly. heat in the pipe before and after the diverter valve is the same (not hot at all) so I don't think it is this, hot water does get properly hot when immersion is switched on though. Could this be faulty tank stat or room stat? or both? What I can't understand is if one of the stats are faulty why is it affecting both hot water and rads? the boiler is an ideal and is only 4 years old, pump, stats, diverting valve are all also only 4 years old, timer seems ok. system has a cold water feed to a tank in the loft and all rads have been bled to no avail, any help will be appreciated.