i would go for a 30kw ideal logic plus which has a 5yrs warranty and having fitted these boilers you should'nt be disappointed,i said 30kw but this depends on your hotwater demanded.
yes if you order 173624 from parts centre this has the housing and cart all in the kit,have you disconnected the div head and tryd pushing this cart spindle in and does it come out on its own or is it stiff,
HAS YOUR ENGINEER RECHARGED THE VESSEL TO 1BAR TO MAKE certain THIS IS THE CAUSE OF THE PROBLEM?
IF SO AND THE VESSEL IS FAULTY IT CAN BE REPLACED BY REMOVING THE BOILER OR A secondary EXPANSION VESSEL CAN BE FITTED TO THE SYSTEM.
i would go for a boiler with at least five years warranty,so that gives you no unexpected repairs and peace of mind after paying out for a new system,just fitted the new ideal logic plus combi/5yrs warranty good looking and easy to control.