ferroli optima, 900 hot water problems

[smtplumbing]

all right guys my pal has been at me for donkeys to come and fix this boiler, i normally dont do breakdowns but i thought i better try help him out. His water in his basin goes hot cold luke warm cold hot. the boiler fires up ok no problems when call for dhw is needed it just does not retain the heat temp. when the bath is run its ten times worse which is a pain for having a nice warm bath.

Heating is fine, the thermister near the drain off on the hot pipe has been changed. anyone deal with these boilers cause ferroli would not other bother helping or maybe i just got some rubbish lol

Thanks steven