Help with dripping overflow pipe - Open Vented System !

I have an old back boiler.

I have just turned the setting up, and have also turned the setting up on the pump to make my radiators nice and hot, which has worked well.

Ever since doing this my overflow pipe is dripping fast, this I believe is coming from the feed and expansion tank above the cylinder in the bathroom, however there is another insulated coffin tank above this in the loft which I believed was for overflor purposes ?

I don't think that there is a problem with the ballcock in the feed and expansion tank, but believe that this is being filled by an overflow pipe coming from the cylinder itself (upside down j shape).

If I turn down the boiler and pump settings the overflow problem stops, however my radiators are not hot enough !

What can I do ?

Thanks in advance....

may not be enough dry head on the vent - and the pump may be too close and so pumps over a bit when turned up.

just leave the pump speed as it was and see what happens

As jackcole said above, expansion pipe may not be high enough over tank and your water level may be too high in F&E tank.
Check here //www.diynot.com/forums/viewtopic.php?t=268548 to see further details.