Today I spent most of the day troubleshooting my Endurance EBP0175 boiler.
I've been without radiant heat since Christmas eve and was reminded how nice radiant floor heat is.
Turned out to be a stuck flow switch on the domestic side; still not sure why that prevented the boiler from processing signal from radiant side while domestic was working without a hitch.

All in all, a very satisfying day with no need for new parts.