Back Claiming WFH Tax Relief?

[Mr_Yogi]

Sorry if this has been asked before, but my search didn't come up with much.

I'm PAYE, and kept forgetting to claim the £6/week tax relief for WFH during lockdown. Prior to covid I was working in the office, and my empolyer has not provided any expensis for WFH.

I've gone through the HRMC microservice and trying to claim for previous financial years doesn't present any obvious options for where I can claim. I've found a number of walk-throughs showing how to claim for the current financial year where if asked about when you started working from home, and then adjusts your tax code, but the options available for previous years don't seem to cover it. Does anyone know which section you need to claim under, and what what values to use? £312 (52 x £6) or the 20%/ 40% of that?

Also I was working from home due to COVID from the beginning, before 6th April 2020, so can I claim for 2019/2020 financial year at £4/ week, or is that only under certain circumstances?

Thanks for any help.