Citroen C5 2012

My black Citroen C5 is a large family car. It is manual, has air-conditioning, runs on deisel, with power windows. The interior is in nice comdition, no smoke or pets. There is a mechanical fault, that stopped me driving this car a few months ago. I think it is a diesel injection problem but i am not mechanically minded. The car requires a new battery, but when i jump start the car it starts immediately even after a month and idles fine, but it doesnt have power when driving so cant maintain any driving speed. Before this fault, it drove fine, front tyres are new. It is due for service and still registered until november. I have another car i use so i don't wish to repair this one. I hope it suits someone with mechanical know how, or interest to fix it as it would be shame to use for parts only.