Bosch 532i Hot Water overheating?

Fault seems to be intermittent.
Had local engineer in who fitted a new high temperature sensor last week.
Everything OK for a few days but problem has reoccurred and engineer is on hols.
Heating system is fine but occasionally when hot water tap is run the pressure rises above 2 bar and temperature climbs over boiling point and bangs. System shuts itself down until temperature returns to normal.
I have switched on ECO mode which seems to improve things a bit but not all the time.
Any ideas please?