tips on replacing three way mid position valve

[norfolkinesox]

i have a back boiler sysytem and ive recently had problems with radiators getting hot when only hw selected,then somke days later i couldnt get ch working when selected on own,replaced the motor and seems ok,but the valve only struggles to go fully over to the hw postion when selected and needs a bit of help manually(a little push and moves on own),and the radiators are nowere near as hot now when only hw selected, which tells me the motor was knackered but the valve is still passing slightly

looking at replacing valve whats the best way please and thankyou

 

[bengasman]

without a doubt, the best is to replace the whole valve.
10 to 1 or thereabouts that your system is soiled as well