Duracell make a replacement for the BP511 under the model number DRC511 in the UK. I bought one ten years or so ago and finally had to replace it a couple of years ago. I was using it as my main battery in a 50D and taking about ten thousand images a year. I've replaced it with another Duracell DRC511 and that also seems to work well. A company called Duracell Direct sells them in the UK for £14 including delivery. I found dealing with them straightforward and very speedy.
Used Wasabi and Digipower and Energizer replacements for the D50 (and my G6) back in the day. Funny this came up because I just came across some of these cleaning out my "parts" drawers . And, came across two chargers! Seeing if they will still hold a charge right now. If so, will see if anyone can use them.