baxi genesis 80?

intermittant fault: when using hot water only,there is none. so c/heating must be put on, then hot water works. However now nothing is working.Pressure is fine. im a corgi engineer, but have little experience on combi's.
told by baxi technical 2 different opinions.1 pressure switch 2. circuit board,
as job is a favour for friend I dont want to order wrong parts that wont be of future use.