bypass ballansing cock

I have Glow-worm Iconomy Plus boiler 6-7 years old and fully pumped S Plan system about 30 years old.
I have 5 radiators, one of them is nearly cold.
Recently my boiler got noisy and I've strarted reading different forums in order to sort this out. Last night I tweeked with the bypass cock, I closed it and then opened it a little bit (something like half turn) My boiler got noisier but the cold radiator became relatively hot.
My question is: is it OK to leave the bypass cock like this. If not, is there some general rule how to set it (I cannot remember the position it was before).
Also I guess my system has a lot of sludge and scale thus I plan to put some de-sludger and then drain. Should I tweek with the ballance again after this?
Thanks

Stop twiddling! Your system sounds like it is full of crud and air due to lack of inhibitor. Your boiler may be noisy due to buildup of scale if in hardwater are. Ensure you have correct cleaner for your system and follow instructions properly. After that put in correct inhibitor. Then bleed all radiators.
Your bypass is solely to protect boiler in case all your rads and immersion tank are turned off. The object is to allow residual boiler heat to be pumped out so boiler does not overheat. Therefore it should be on sufficiently so boiler can cycle without kettling.