WB 42 cdi hot water?

I seem to have air in the hot water system, the heating is bled and works fine, the boiler was working fine till i checked the pressure was ok to go on holiday and leave with parents. the pressure was at zero so i topped up to 1 bar, now when i run hot water the temp display rises very quickly to 99 and cuts out leaving the display showing U1 going up to U9. the hot water initially come out too hot, then soon runs cold as the above happens.