Wow thanks guys, all the advice much appreciated
I've never joined a frequent flyer program before and let I've done lots of flights over the years...Never knew there was all these ways to earn extra miles, I always thought you just had to fly lots (which is why I never joined)!
So...I've now joined the diamond club as per the advice. Saw there was a box for voucher code on signup, so a quick search, and using code 1JA1KD seems to have got me 1000 destination miles in the account
Next, onto the bmi credit card!