no hot water - again!

[suff]

I thought I'd found the 'cure' to this - but it appears not. Potterton gravity hot water system - 2 port motorised valve (with new motor) - tank stat fine. System will 'spark up' - but intermittently. Water reaching the tank clearly hot by feel of pipes but not nearly so hot leaving the tank at the top. Heating engineer reckons boiler is fine but system needs complete new valve (& adjoining pipework ) necessitating draining the tank etc. - cost about �200. I'm just not sure this will make the difference. Any ideas out there?
Suff
ps CH system fine.

[Hard@it]

do not follow this , if your tank is heated by gravity water and your header tank is full, and your stat is calling for heat then your two port valve opens - then you should be getting DHW. Why don't you manually open the two port valve and keeping the heating side off, and not running your taps check that your boiler stat is giving enought heat and your tank stat is set properly - you should get more than ample DHW. It sound like there is something wrong with the controls side or your stats are not set high enough !
Or have I got this all wrong Hard@it

[suff]

Thanks Hard@it - sorry about delay in responding. I'm completely baffled. The stats on the boiler & tank are set OK (3-4 on boiler & 60-65 tank). Am I right in thinking cold water from the header tank enables hot water from the boiler to rise in the tank (& then out to the taps when turned on)? How full should the header tank be?

Cheers

Suff (ering)