I’ve kept old batteries in a little plastic bag for recycling. I now see that some have become corroded. Is it better to seal them in a bag and chuck them out with the general rubbish or leave them out with my other recycling?
Take them to your usual battery recycling point. The fact that they're corroded won't matter. Definitely DON'T put them out with your general rubbish! https://www.leicestermercury.co.uk/news/local-news/refuse-lorry-catches-fire-after-7614562
You should not dispose of them in household bins.
Batteries need to be diposed of correctly either take them to a council recycling centre or take them to a shop like Wilkos or similar that has a recycling bin specific for old batteries.
Weird ain't it. Before all this recycling became the done thing, folk slung used batteries into the dustbin, and one can not recall any fire reports being caused as a result. Perhaps they make batteries differently these days.