ok... looks guys changed the dirverter valve never removed the air from the system!!
- and I removed some of the air the same night (had to do couple of time as presume air/water is all dissolved due to CH running and needed to wait when boiler hasn't been started and allows air to trap in a radiators- been freezing past few days).. so presto! as soon as got rid of air, CH started again .. could that be making CH cutout?
Well.. CH is working again (thank god!), but DHW has the same problem (on DHW demand - CH supply gets quite warm! so all heat is being discharged in CH)
HE is not in stock means going to be next week by the time heating eng replaces it! - hope this fixes the issue before something new appears!