Xsara Flat Battery now low idle

I have a 1999 Citroen Xsara West Coast 1.4i. I went to start the car yesterday and the battery was totally flat. I tried to start the vehicle on a couple of occassions. I have since started the car using jump leads from a second vehicle. The car now runs fine and the battery has regained charge. However, despite all seeming well, the Citroen's rev-counter is indicating that the car is idling at c. 300 - 400 rpm rather than the more usual c. 900 rpm. I am confused as I say there are no other problems and the car drives fine. Could I have damaged the rev counter indicator when jump starting? The rev counter appears to function fully and correctly when driving.

Thanks in advance for any help.