OK Welshy - I'm a girly so I had the same problem with height/weight but on the other end of the scale (ie: I was too light). I solved it by drinkinng a gallon of water five minutes before my medical then putting on the weight pronto.
The official statistics for the RAF are:
Weight - between 56.8 and 94.0 kg.
The cool thing is, your height to weight ratio isn't important, you just have to be inside those boundaries above, and also be between 865 and 1010 mm height whilst sitting down. I have the official stats here on my lovely green medical form.
Need any other tips, email me, as I've literally JUST passed selction and so have all the up-to-date stuff to hand.
Regadrs
YakYak