Not charging hoover fd22 (Addiscombe CR0)

At first this started as a battery replacement, but the new battery only lasted a month, so I assumed it was just a rubbish battery I'd bought. But when I put in the old battery (which had been fine, didn't last long) it wouldn't charge. I've tried with a replacement charger and the light flashes on and off. Does someone want to have a go, or use it for parts?