Citroen Berlingo 2007 1.8 Diesel van - Engine fail light intermittently on?

My Berlingo van has started 'spluttering' at revs over 2400 and the Engine fail light illuminates for a few seconds and then goes out. Overall it is now 'gutless'. I haven't run the van for 3 weeks and believe supermarket fuel / injectors / condensation may have been causing the problem. I have put in 40 Lts of Main brand Diesel and Diesel injector cleaner Redex and as this has made no difference, how do I drain the possible condensation out, or can anyone recommend another course of action. The light seams to illuminate on Downhill stretches or slowing down. Coasting helps to put fail light out after maybe 5 Secs. Van worked perfectly before stopping work for 3 weeks.