Remove and clean out the piping with the flexi hose in the middle of it leading to the main heat exchanger.... I had the same issue and after removing that pipe work i found it was quite clogged and had reduced bore... And good clean out of that pipe work and flexi along with changing the nts...
I've just solved this issue with my ecomax 824/2e... I had already replaced the divertor valve, cleaned and descaled the secondary exchanger and then replaced the pcb... These didn't solve the problem.
Today I replaced the NTS sensor and removed the pipe work with the flexi hose in the middle of...