How much time to allow?

Don't tie yourself to a specific train. Anything can happen.

At that time your train from St Pancras will be off peak. Plump for any off peak evening time for your ticket. These are usually transferable to any off peak train on that day. If you’re using train line or any similar site it says that. Hope I’ve understood your question correctly.

Do you man the Underground or the Heathrow express?

mean not man.

Piccadilly line from Heathrow to Kings Cross should take about an hour, St. Pancras is adjacent to Kings Cross you can walk it easily.
Heathrow Express is expensive and only goes to Paddington a long way fro St. Pancras.
You’ll need to factor in how long it takes for luggage collection and customs.

As the others have said buy a flexible ticket just in case.

If you’re going on the tube don’t worry about buying in advance. You can get a ticket from the machine or just touch the yellow reader with your contactless credit card at both ends.
Like Tanith said, go direct to Kings Cross. You only have to cross the road to St Pancras.
BTW the toilets are free at St Pancras, but you had to pay at Kings Cross last time I was there!

I understood it was the train from St. Pancras the op was asking about not the tube.
I agree best not to book a specific train, if your flight is delayed taking off you'll be scuppered.

As others have said you don't need to buy Tube tickets in advance.
The unknown time will be from whether you land on time, through customs and luggage pickup. And shuttle train to Tube station.
I did that journey once, returning from Kuwait. No problems except the Tube was packed.

Yes now Nina, I woke upon the night and thought Oh she means the train at St Pancras. Doh ?

Allow 3hrs from touchdown OP or more if you’re the worried type like me. Or just buy the ticket at St Pancras when you arrive.

Thanks for you replies. I am looking at Trainline, which is giving times for my whole journey., i.e the time the tube leaves Heathrow Terminal 2 and 3 rail station into St Pancras, and the time my main train leaves St Pancras for home. So if I land at 18.40, Trainline says my journey north can start at 19.20, 19.50, 20.46 or 21.54. So factoring luggage collection and customs, which train should I opt for?
"Specified train only" tickets are the cheapest of course. "Super off peak" tickets are three times the price. But does that mean I can catch any of above train?

But what would happen if you were delayed and didn’t at the specified time? It’s up to you if you’d rather risk it.
You can catch any train after 19:15.

‘Land’ at the specified time.

I have been in Heathrow and waited 15 minutes for my luggage, and on another occasion it was over two hours! You really cannot rely on a specific time.

There is no way you can make those earlier times. It takes at least an hour on the tube to Kings Cross, sometimes a bit more and you then have to walk across to St Pancras and go upstairs to catch the train.

And of course disembarking can take a while, then passport check, baggage reclaim, walk to the underground or the shuttle.

You might make the20.46 if everything is slick and speedy.

Sometimes if specified train is really cheap I take a chance and just accept I will lose £5.00 or whatever if I don’t make it and then have to buy a standard off peak.

If you are set on the specified train I would go for the latest one myself.

I agree with all that has been said . For future reference, the TFL website will tell you exactly how long any journey across London will take and give you the best route.

Trains and planes can be unreliable for all sorts of reasons. I would have thought an open ticket is best and would help to reduce any panics

Flexi ticket for train is best, planes can be late in arriving.