intermittent hot water ?

I have wall mountd worcster 30cdi - 3 yrs old .CH is working fine. the hot water supply pipe runs between dwn stairs ceiling & upstairs floor. It supplies -in order -ensuite basin tap ,power shower,bath tap ,basin tap then goes dwn stairs to cloakroom basin,utility sink tap then kitchen tap. If I turn on any hot tap *except bath tap * I get hot water , if i turn on another tap (except bath ) i continue to get hot water . however if I turn on the bath tap first I only get warm water ( a few second of hot at first ) if I turn on another tap the water remains warm from both taps , if I turn off the bath tap I will get hot water out of the other tap i opened . This has been happening for abt six weeks .I have had DHW sensor changed but no change . The only thing I can think of that may have happened to boiler abt six weeks ago ( but not sure exactly when ) the pressure was toppd up to 1.5bar instead of manuft recom 1.0bar . I droppd the pressure to 1 bar abt two weeks ago with no difference _so not sure if this even connected . Eng talking abt changing dhw exchanger plate ! (big job & expensive ! ) Anyone got any other suggestions plse .