On Jul 13, 4:09*am, "
wrote:
Would it be possible just to add 2.40 or what ever the fare to my
oyster card?

Yes - though London Underground now impose a GBP5 minimum top-up when
using their ticket offices (i.e. the ticket windows), so you'd
probably have to top it up at a self-service ticket machine at
Heathrow tube station and you'd need to have the correct amount of
British coinage to do so, as they won't give change when topping up an
Oyster card with cash - note that some Tube ticket machines only take
cards, so make sure you don't join a queue for one of them!

Also there are still a few Tube ticket machines out there that don't
'do' Oyster at all and only sell paper tickets - not sure if this is
the case or not at Heathrow, but you can avoid them by looking for a
machine with the yellow Oyster pads, like this one:
http://www.flickr.com/photos/anthonyfalla/3089886764/

There's also a TfL London Travel Information Centre opposite the
entrance to Heathrow Tube station - I'm not sure if they have a
minimum top-up amount policy, but they're normally pretty helpful so
they might be willing to add the amount needed correct fare to your
card and give you change - I couldn't say for sure though.