thetrainline.com used to be one of Sir Beardy's moneyspinners, but I think he hived it off a few years ago when the profits dropped from obscene to marginally less obscene.

There is no National Rail station at Heathrow, hence the potential aggro as I mentioned above.

In order to use the two for one offers, both people must have valid rail tickets (Travelcards are accepted because they are good for carriage on National Rail services in the paid for zones) for the day they plan to use the offers. Again, a Travelcard loaded on Oyster won't be accepted, nor will a paper Travelcard from an Underground ticket office.

Won't repeat what I said above, but after having crunched the numbers I do think that a seven day Travelcard will be the best choice, plus using PAYG on Oyster for the journeys to and from Heathrow and central London. If you buy a seven day card to start on Sunday morning it will be good until close of service on Saturday night, basically 0430 Sunday morning.

Whatthehell, once more with feeling. Oyster PAYG fare from LHR to central London on the Saturday of arrival: £2.20 (cash fare £4). Walk across Green Park from Mayfair to Victoria (or take the bus - £1 PAYG on Oyster) for paper Travelcards at £25.80. Let's say there will be one trip to Kew Gardens during the week - if you spend an extra five minutes popping out through the barriers then back again in a zone 2 District line station you can pay £1.10 for the trip to Kew (cash excess fare £3.20). Journey from central London to Heathrow on a Sunday is £2.20 (cash fare £4).

So £25.80 plus a few extras ~> £32.40 plus the ability to use those paper Travelcards for the two for one offers. If you get seven day Travelcards loaded on Oyster you'll have the same total in fares but no two for one offer use. If you pay cash fares to and from LHR, pay cash excess fare to Kew and use paper Travelcards whilst in the city you'll pay £25.80 plus £8 plus £6.40 (to and from Kew and zone 2) ~> £40.20.

You will have to allow enough time to return the Oyster cards to the Underground station at LHR before you make your way to the terminal building, as you'll then receive the £3 deposit plus any unused PAYG money back when handing in the cards.