I have had to replace a digital roomstat. However, the wiring is not
the same as the old one.

I have now got thoroughly confused about what I am trying to achieve!

Roomstat had L, N and 'Call'.

Boiler of course has L N, and also a connector for a room stat (space
for 2 wires with a diagram indicating a switch). Doesnt help that the
two wires there are both black!

To use an external controller, the boiler has to be swirched to
'constant' for heating.

I assumed that basically one side of the boiler connector would be
live, which would power the controller, and the other side of the
connector would get the output from the stat. So one side would be
live, the other side 'may' be or not depending on the temp.

However, both sides are not live until the stat calls for heat and
then both sides are. But, then the boiler doesn't fire...

Sorry it's such a long post!

David