This should go directly to the offer. If not try this link. You would just enter 1 assuming you want one set of 5 tickets. These are not the $85 seven day tickets but are five days instead. The 4 tickets you purchase are $99.95 each for a total of $399.80 which comes out to about $80 a ticket so your only saving $5 per a ticket over the $85 tickets. Unless you need 5 tickets I would think about getting one of the other deals.
Not sure what the ages of the 6 individuals are so heres a couple of different possibilities. If you have any children between 3 and 9 and are going 3 days or less you could always use the buy one get one free deal from the international section of the Universal site. Its $112 for one adult and one child ticket. If you have just one child between 3 and 9 its $452 (112 + 340 (4 $85 tickets)) two children its $394 (224 + 170) and finally three children would be $336. Using the Mastercard deal you would pay a total of $485 (400 (mastercard) + 85).
One last option is also from the UK section of the international site and also requires you have some children going. Its a 5 day ticket which includes wet and wild and is also buy one get one free. That is $130 for one adult and one child. The tickets are still cheaper then the Mastercard deal, are the same length, include Wet and Wild, and you have a 14 day period to use the days unlike the Mastercard ticket.
Just want to point out both of the buy one get one free offers require that you purchase them by April 30th and use them before the end of the year.