I highly recommend you transplant your hydrangea into the ground - a nice partially sunny spot will do just fine. Use mulch to retain moisture. The name 'Hydranga' means 'water loving plant'. Do you see where I'm going? The pot cannot hold enough water to satiate a hydrangea's thirst. They are deathtraps. It becomes yet even more thirsty as it grows bigger, nevermind NPK. I suggest you plant it in the ground.

Hi there. Hydrangeas don't do well in pots. It either needs to go into a very large container or (ideally) into the ground. If this isn't possible, I would either give it away or sell it (Gumtree works for me), and replace it with something which will be happy in a pot.

Hydrangeas don't usually suffer from iron deficiency. Its more likely a more general lack of nutrients. Have you been feeding it? A bigger pot or the ground is best as above but give it a feed now while you're getting that organised.

Thanks for your replies,explains a bit ,would all round fertiliser be best then,in the ground it will go

If you are going to plant it straight away there should be no need to fertilize it. But if you want to why not try a foliar feed - put the mixed fertiliser into a spray bottle and spray the foliage with it. Surprisingly it can be absorbed by the leaves and can give the plant a quick lift. Its best not to over fertilize at this time of year for fear of encouraging a lot of new growth which wouldn't have enough time to harden off before the cold weather.
Plant it with plenty of space from other shrubs as they get large in time.

Remember that a blue hydrangea planted in alkaline ground will go purpley pink and a pink one in acid soil will turn blueish.

Thank you everyone l think it's destined for the ground