I deferred the July 2020 POA 2nd payment but paid the 1st POA in January 2020.
As they are the same amounts will the net impact on the 2021 self assessment be zero. i.e. I will not need to pay the 2nd deferred payment from July as the first payment will cover it?
Not 100% sure exactly what you are asking here - technically payment on account still needs to be paid - albeit as with every year when your final tax bill is calculated there is a chance both payments on account may have been or could be revised - eg if tax lower than the sum of the two payments due.
You will need to finalise your year end tax bill asap to find out if not already done
example you calculate 5/4/2020 bill as being £1200
you paid first payment on account of say 500 last jan
second payment of 500 will need to be paid 31/1/2021
balancing payment of 200 will also need to be paid
If your ye tax 5/4/20 bill was only £800 second payment could be lowered to £300 necessary to clear down that year tax
Ref 2021 nothing changes and payments on account are calculated as normal with reference to ye 5/4/2020 tax bill . In above case if it was bog standard you woudl have £600 to pay
So potentially you have full second payment on acount + posisble balancing payment both ref ye 5/4/20 and potentially the new payment on account calculated the same was as if the second payment had not been delayed.
Note some individuals who have had payments on account and a balancing payment that was requested to be coded out may find hmrc computers have done funny things with regard to second payment on acount and tax coded out - these people should contact hmrc for help/clarification