How long does it take from arrival at t5 to get to the arrivals hall with baggage?

When you land you need to walk to Immigration where your pport will be inspected and further questions will be asked to determine the purpose of your stay in the UK. You then need to reclaim your baggage at the Baggage Reclaim Hall then you p through Customs. After ping through Customs I presume your taking the Heathrow Express train to Paddington Station, then the Tube to Kings Cross? If so you must allow extra for this. I think you should get the 10:00am train from Kings Cross to be safe.