How can i fix my Citroen Xsara 2.0 HDI XL?

I have a Xsara 2001 plate 2.0 HDI XL Hatchback.
When i went to start the car the battery was very sluggish and eventually i had to call Home Start rescue out. They jumped started my car and once i had taken it for a good run it seemed okay.But 4 days later i noticed quite a drop in power i was struggling to get 40 miles per/hour at 40K revs. I have tried adding diesel Redex but that made no difference, i have checked my owners manual but that just advises me to visit my Citroen dealer which at the moment i cannot afford! I have no warning lights or code display at all so i am not really sure where to start.
I would be very grateful for any ideas. Thanks.