The easiest way to get to the apartment from Heathrow is to take the Elizabeth line to Whitechapel and then the Overground (northbound) to Haggerston. 

From Haggerston Station,  it's about 5-7 minutes walk to Hertford Wharf (put N1 5QS into your map app).  

It's worth asking about any disruptions in service before you set out, especially at the weekend.

Enter the postcode (N1 5QS) on Google Maps to find the exact location.