Fuel problem in Citroen Dispatch?

Dispatch HD 2002. Went to have exhaust changed and on collection it turned over, started then 'died'. Engine stop light on but engine turning over. Changed Cam belt because they thought it had jumped a tooth and van runs good for 3 weeks. Short journey up to the local shop, okay, on return van starts and drives a few yards then 'dies' but turns over as before but won't start. AA arrive after about 2 hours and the van starts no problem. Runs again for a few weeks then the turbo fuel pipe comes off... AA fix that and again okay for about a month. Set off from cold and drove about 1.5 miles; changed down from 5th to 4th and starts hesitation; change to 3rd and get power back; up again to 4th and it hesitates and 'dies'. Turns over and over but won't start; wait for an hour and a half and it starts okay!!!! Any ideas? Quite often the exhaust smoke smells rich - tried Redex to clean out. I think also the problem seems to happen when just on a quarter full of fuel (might be coincidental).