Just checked my account again and the £200 of Promo Fares (4 x £50 F returns) have now credited the Reward Points to my account. ^
I now have enough points for an F redemption for 1. I could hold off till I get to the 900 points for 2 tickets but it looks like the 500/900 rate is a promo so maybe better to grab the 1 x F ticket voucher now and it is valid for 6 months.