Freelander TD4 engine wont start

For the past couple of months now my diesel freelander has been causing problems. A few months ago the engine warning light came on. Then a couple of days later it went into limp mode. I took it to a garage and after reading the error message they replaced the Mass Airflow Metre. This fixed the problem and the freelander drove great but only for a day when it just suddenly died on me, all the dash lights on and it just wont start. So today it's had a new remote fuel pump but that made no difference, it still wont start. Then last week we decided to replace the high pressure fuel pump and it worked!!! We drove it fine for a couple of days and then this morning the engine managment light is on again and it wont start!!! Please any ideas as it's causing us a fortune and no one knows what is wrong with it and the error codes as so general. Cheers.