Read this - it's important - Do not try to keep starting the car.
https://the75andztclub.co.uk/forum/s...d.php?t=106447
As for a new battery try Tayna - good reviews and after reading about them on here I've used them a couple of times and can confirm their good service.
https://www.tayna.co.uk/Rover-75-Car-Battery
As you've a diesel I'd probably go for the Varta E44 with a 5 year warranty.
Good Luck.
Andy.