Wow, that's some spend rate you have! As you are accumulating at that rate, I would seriously recommend you consider also building up a balance in British Airways using the BA Amex card.

If you want award flights to the US, for example, you are restricted with BMI / Star Alliance - currently no *A flights from LHR to JFK, for example, so you need to transfer. Similarly, with BA, it is very difficult to get seats to Asia whereas they are relatively easy to get with BMI / *A. Having balances in both airlines gives you a wider range of options. You can also get one-way awards with both BA and BMI, so (for example) if you couldn't get the return dates you wanted via Diamond Club you would also have the options to consider a one-way with DC and the return with BA Miles. Its also worth remembering that BA has a fully flat (horizontal) bed in business class, whilst most Star Alliance carriers have the less comfortable sloping bed in J meaning you need to book First to get fully flat.

Its also worth considering whether it would be useful to get a generic Amex Platinum Card. You would earn 1 Membership Reward point per £1 spent, and you have a wide range of transfer options, including Hilton HHonors (1 Amex = 2 Hilton), Priority Club (1 Amex = 2 PC) and BMI (1 Amex = 1 BMI), plus various others. At present you may run the risk of having too many airline miles and too few hotel points - although I accept that if you are a Four Seasons / Ritz-Carlton / Mandarin Oriental sort of guy (as I am) then hotel points won't be much use to you as none of these chains have a loyalty scheme.