Hot water tank size / heat pump

What temperature do you expect the heat pump to heat the water to?
What is the net usable hot water in a 300 L cylinder?
What is the heating capacity of the coil in the HW cylinder
Can you post a link to the Prt L doc where you did the sizing?
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https://steps.ie/cmspages/getfile.aspx?guid=b410fe27-2f49-4a47-9a16-7065752f18f4&forceattachment=1
shows how to do the math, assuming HW temp, cold water temp and shower temp.

for 60/10/40 5 ten minute showers will require 250 litres of HW.

IMO the HP alone will not do the job all year round

Whats the spec on the heat pump, can you share it?
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Can you please find the 300L reference in Part L and tell us what page it is on in what document?
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You may have hot water at 60 but it wont all come from the HP, especially in winter.
Most of them have an elec element which is used to maintain the 60 output.
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So you might have a 9 kW element in the heat pump to take up the slack.
A better idea is to use the HP to bring it so far, maybe UFH temp and then have a night rate immersion in the HW cylinder .

as for 220 at 60 yielding 300 at the tap does not make sense if you look at the link I sent last night.
15 at the shower is c 9 hot and 6 cold
Its closer to 370 which suggest that its not based on 60/10

Our heat pump (joule samsung) heats water up to 48 degrees. When the temperature drops under 43 degrees in the tank, the hp starts to "top up" the water temperature in summer months (top up happens as soon temperature drops under 48 degree if the hp is already on heating rest of house in winter months). So the total available hot water is technically greater than the size of the tank as the hp is topping it up as soon as needed, and not once a day.