If as a result of Coronavirus have been/or are requested by your employer to work from home before the end of the tax year, you are eligible for up to £124 tax relief, even if that is for just for one day!

HMRC have decided that it’s simply just too much of an administrative nightmare, with millions of people sometimes working from home and sometimes not, to ask for monthly proof of expenses, so they have made a blanket decision for the 2020/21 tax year, as they confirmed,
"We recognise that the working-from-home situation is very fluid this year, so we're accepting claims for the full year's expenses. That includes even if people have only worked from home for some of the year, to avoid needing to contact us if you have to work from home again."
This means if you worked from home at any time since April 6th 2020 and before 5 April 2021, then there’s a simple way to claim back either £62, (basic rate taxpayers) or, £124 (higher rate tax payers). For clarification, if you were furloughed you were not working and this does not apply.
You won’t get the money paid into your bank, but instead will get either £1.20 or £2.40 deducted from the tax you pay per week, as your tax code will be adjusted within a few days after you log on to make the appropriate change.
If more than one person is working from home from the same house or apartment, you can both claim.
However you cannot claim this tax relief, if you actually chose to work from home, or for a period where you were furloughed.
HOW TO CLAIM THE TAX RELIEF
The online service will automatically apply the whole 2020/21 tax year's relief via your tax code, making claiming the whole amount very easy and anyone who hasn't already put in a claim, can use it. You can use it to make a claim since lockdown started on 23 March 2020, just put that date in.
If you do that you'll get a refund for the whole of this tax year (which started 6 April 2020) and two weeks extra on top.

You need an existing Government Gateway ID, or to create one as part of the claim process.
In the online form, there are two key boxes
Amount paid by you.
Enter £312, even if you've already claimed for any amount, you still need to put the full £312 amount here.
Amount paid to you by your employer.
Enter £0 if you haven’t been given a work from home allowance or already claimed your expenses back from your employer.
Confirm you are eligible here and make your claim
If you do a self-assessment form you can’t use the online tool.
HMRC has confirmed that when you do your self-assessment form you will be able to automatically claim for the entire year, in the same way as those using the online service.