Just discovered that my CH system pumps over in spurts into the expansion tank. Not continuously but enough to warrant attention. The easiest option for me is to extend the upsided-down J ...but..will this work?  How high does it need to go as the roof apex is very, very low. ....probably less than a metre above the expansion tank water level.

The boiler is an elderly Worcester PJ26-32.  There are four pipe connections at the back.   CH/HW flow is top left and flows into the pump. Bottom left is for the main CH/HW return. Top right is the overflow/expansion pipe and bottom right is the feed pipe from the expansion tank.  So my understanding is that it is on the correct side of things so why does it pump over?

Checked and water flows freely from header tank into boiler.

I've seen this before in lofts where there is low headroom and it happened when the pump turned off.  There was a slight surge and about a cupfull of water pumped over.

We turned the pump down from 3 to 2 and the pumping over stopped and fortunately the heating system still functioned properly on setting 2.

If it is just a short surge from the pump turning off then extending the vent pipe will definately help as the 'surge' water' will have to travel further before exiting the vent pipe.  

If you are really stuck for headroom then increase the size of the vent pipe from 22mm (which I assume it is) to say 28mm or bigger as this will have the capacity to contain more of the surge water making it less likely to pump out.

In EXTREME cases you could even construct a manifold from approximately the header tank water level and have several vent pipes!

Anything to be able to 'contain' the surge water for the few seconds it is....er.......surging?

You know what I mean