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Hello form Italy!

I am planning to come to London at the end of the next week, going also
to Oxford; so I was checking some cheap buses (instead of train) to save
some money.
I found out a very cheap fare in the Megabus site, but I have a couple
of doubts.
I read somewhere in the Megabus site that their London-Oxford service is
provided by Oxford Tube. I checked the timetables in both OxfordTube and
Megabus sites, and they are slightly different.
Besides,the departure and arrive are at the same place, but while the
OxfordTube has some more options to hop on and off (including a stop
around Marble Arch which would be better for me), Megabus in London just
lists Victoria.

So, are the above two completely different services? If they are the
same, is Megabus timetable correct?
In any case, do you know if Megabus has more stops in London?

(I asked these questions to Megabus contact address - something like
megabus.enquiries at stagecoach dot com - but had no useful answer:
they told me to phone an Oxford number, I wrote back asking if they
could give me an email address, or (better) kindly found this info
themselves, but I had no more replies in more than a week...)

If you have any other useful info about London-Oxford buses, please
post!

TIA & Ciao
max