Potterton EP3000 -> Horstmann C27 boiler programmer wiring
From the Horstmann HEATING PROGRAMMER REPLACEMENT AND WIRING GUIDE.

EDIT
I think the diagram you have, is showing the internal view of the programmer (simplified of course).  In this image the blue rectangle represents the internals of the programmer, and the red lines represent the lines going to the boiler and power.

The lines with arrows are probably switches.

And if both HW and CH were on, it would look like this.

Because of this, 5 has to be linked to L to supply power to CH, like so.

But I'm guessing the new programmer supplies power to CH internally, so with the new programmer it should look like this.

Again, I could be wrong here as I've never worked on a system exactly like yours.  After looking at the wiring diagram the arrows are either switches, or diodes.  Though, diodes would look more like this -&gt;|- .
Hope this helps.