how can i fix my halstead ace 30 boiler?

i have recently had my boiler replaired after it was showing error 1.no hw or ch for a full week! but now is working fine but it is leaking hot water, i have been told it is the pressure, when the ch is on pressure gauge goes upto 3 is this normal and how do i release the pressure??
i have called back the engineer you recently fitted the part and he released a load of water from the actual radiators, but this is happeneing every time my heating is on for a while.
any help would greatly be appreciated and save me paying out further to call out an engineer