You can see here, the second coil (primary) has now been connected up and the lower insulated pipe coming in from the left of the photo is the feed and expansion pipe from the header tank.
Shown here is the second Grundfoss pump installed in the towel rail circuit. With the towel rail immersion element doing it's thing, hot water can now be circulated through the spare primary loop in the calorifier and thus transfer heat to the secondary. With no solar input from the collector panel, I can still generate a few litres of warm water for washing up or handwash.
I just need to get in among all the gubbins now and tidy up all the cables, clip the pipes into place and finish everything off. I still need to fit a frost stat so the system only kicks in when the temperature gets too low for comfort and a digital timer so I can 'program' the heating system.
I shall update my diagrams and post when I get a chance, but in essence I now have a dual-sourced central heating system. Solar input for the warm sunny days which is just used for water heating in summer (where, in theory, I won't want any space heating) and electrical input for the winter which also acts as background space heating in the kitchenette during the colder months. In between times, I can use a combination of both, so I should be sorted now for all states of the weather.