Shopping at Tesco would take more than 6 weeks for the miles to post as it's done on a quarterly basis. The last vouchers came through a couple of weeks back.

gate365 is probably the best bet, although I'm not sure if some merchants are better than others with payment times. I've only used it twice myself.

Also see if there are any spare hotel miles kicking around? They would probably tip across in that sort of timeframe.