I have read remarks about using "Justairports" from LHR into London. Can anyone tell me what it is, or how to get information about it?

It is a car service one needs to book ahead. justairports.com

It is a chauffered car company

www.justairports.com

If you book it, it's cheaper to pay cash rather than with a credit card.

It's cheaper than a taxi or using the Heathrow Express if there are 2 or more of you - and it takes you direct to your hotel

As alanRow says, it works out cheaper than a taxi or Heathrow Express if there are more than 2 of you.
Used them a year ago and it cost us 40 pound from Heathrow to Kentish Town
(quote was 33 pound but they charged extra for waiting time because we took too long going through immigration).

They have a driver waiting for you (holding up a sign with your name on it) and drive you direct to your destination.

Quicker cheaper and easier.

We also used Concorde Cars for our return trip to the airport(our friends booked them for us).

If you book your return trip to the airport all at once, it works out even cheaper.