At no point - in the information provided with my card, on the website,
nor during the phone call to the number provided in the email I
received when I paid for my second weekly travelcard - did I see a
reference to the word `validator`! I'll look out for them when I next
have some time to kill there. (I got a little bit of a funny look
this morning when I validated next week's travelcard but no-one said
anything.)

On the plus side, if you change your mind and don't wanna renew that
particular week, you can just not validate, and in 7 days the
transaction is void.

The advice I usually give to customers is to visit any ticket office at
any convenient time that it is open, and top up then. If you avoid
peak times, even popping in on a weekend if its convenient at all, you
can just renew it then. Or use the ticket machines, again out of the
peak hours.