Worcester Bosch 28 si II indicator fault light?

My Worcester bosh boiler for some time now has been losing pressure and letting out water from the release valve (the pipe outisde the house. Have had a heating engineer out and and they have changed the expansion vessel, whilst this has solved the problem of pressure I now have a new problem.........When I fire up the heating and it has run for half an hour or so I am now getting the fault indicator light flash at me, whilst this is flashing I am not able to get any hot water as the boiler will not ignite. Once I have reset the boiler and it cools off I am then able to switch heating on and have hot water on demand. Any one have any idea's as my enginner seems to think I need a new heat exchanger and this is very costly to fix and I would be wiser putting in a new boiler, but my boiler is only 5 years old. SOMEBODY PLEASE HELP!!!