changing from train to tube in order to get to Heathrow

Tube goes to T3 but not direct from Charing Cross. Where are you starting your journey from? Perhaps we can find you a better connection point than Charing Cross.

If you DO have to start at Charing Cross, easiest would perhaps be to walk the short distance down the side of the station to Embankment and get on the District Line there, and change at Hammersmith for the Piccadilly Line.

TFL's journey planner (https://tfl.gov.uk/plan-a-journey/results?InputFrom=Charing+Cross&FromId=1000045&InputTo=Heathrow+Airport+Terminals+1-3&ToId=1000105) suggests taking the Northern Line to Piccadilly Circus and changing for the Piccadilly line to Heathrow there, but that would be a more crowded station to connect through.

The journey is starting at Sevenoaks, we're staying near there. Train website indicates you get a tube to Heathrow AT the Charing Cross station. I was just worried about switching at the station, now I don't like any of the options. ??

You're right, it seems Sevenoaks is the most awkward place to travel to Heathrow from. The simplest connections seem to be:

• the short walk from Charing Cross to Embankment / District Line to Hammersmith / Piccadilly Line to Heathrow option discussed above. See it on the map, its really a very short walk from Charing Cross to Embankment, 2 minutes.
• Charing Cross / Bakerloo Line to Paddington / Heathrow Express to Heathrow.
• I think if it were me at 70 carrying baggage, I would take train to Charing Cross / taxi to Paddington / Heathrow Express or Heathrow Connect (cheaper) to Heathrow.

Simplest would be the national express coach from Sevenoaks which only involves a change of bus at Victoria Coach Station

Sevenoaks - Victoria - Heathrow T3

https://www.nationalexpress.com/en

Yes, get off the train at Charing Cross and walk down Villiers Street to Embankment tube. There is a direct staircase from the side of the station down to Villiers Street, which is pretty steep down towards the river, but if you have heavy luggage it's best to come out of the front of the station, turn right, and loop backwards down the street.

The best ideas--The Sevenoaks coach/bus won't work because her flight leaves at noon and the bus arrives 45 minutes later. I think Charing Cross > taxi> Piccadilly> Heathrow train sounds best to me unless any better ideas surface.
We're staying close to Edenbridge/Chartwell and Sevenoaks train seemed most logical.

If it’s a noon flight from Heathrow, it means arriving at 9am or 10am – unfortunately that’s going to be a rush hour journey with the commuters all the way from Sevenoaks. Better start sharpening those elbows now.

I'm finding nothing is cheap and I don't understand why the train website indicated she can get from Charing Cross by tube to Heathrow. The site says 1 change and that's it. Crumb!

I checked out the Heathrow Express and it is costly and discounts don't fit for this situation. So maybe Green Park lift will be best.

I'm thinking that when we arrive, pick up rental car, head to Sevenoaks area, and check out the traffic, etc. we may decide it is practical to just drive her to Heathrow on the day.

Heathrow Express is not the only way from Paddington to Heathrow. My earlier suggestion was Heathrow Connect, which goes on exactly the same route, but stops twice so takes 10 minutes longer, but at much lower rates.

OK!! Thanks all. After checking the tube site I learned there is an escalator at Charing Cross to the Bakerloo Line, change at Piccadilly Circus to the Pic. Line< apparently there's a lift or escalator there> and she stays on the tube till Heathrow T2 or 3 shows up an hour or so later. It costs less than $25 all told.
If there's a flaw in these plans, let me know.

Also note that it is quite a long walk along underground tunnels to get to the Bakerloo Line at Charing Cross - because in reality the station is further down The Strand. Indeed until the 70s there was no passageway connection and the Bakerloo station itself was called Trafalgar Square.