Update for those interested - although it seems there aren't many.
Tickets for the velodrome are sold out for all countries except Brazil. Only Brazilians can buy these tickets but they can buy them for friends. I have now got tickets for 6 of the 8 track cycling sessions via the person I booked my apartment on via Tripadvisor. I have the pdf receipt but careful going this route after much research I would only advise doing this via a renter with a good track record and 20+ 5 star reviews. The owner I did it via is a British (Brazilian resident) so communication was easy and I was able to verify by phone and social media. So off to see Wiggins pick up his silver
but also have via the same source got tickets for the Heptathlon and 10,000m and long jump nights in the athletics.