There are two set's of cable running to the thermostat, one set is the
L, N & Earth, the other is the N & Call.

Does this help any?

It is possible that one cable is from the programmer with the L (actually CH
On, not permanent) and N.

The other cable might go directly to the zone valve, particularly if it
isn't mounted in the same place as the programmer. This would explain the
doubled up neutrals.

Christian.