Hope this helps.
There is a lengthy doc online about commutation, some unusual and arcane rules. Lots of info but worth reading so that your application does not get held up. I found it by searching the defence net. Did you know that there are limitations on what you are allowed to spend it on? Or that commutation can be stopped if you if you are bankrupt or under a Receiving Order. There is also a health requirement. If you bust the rules, you can make yourself liable for income tax on the amount received!
Mine was paid at the same time as my gratuity 2 weeks after leaving. You cannot ask for 'the max', you have to give an exact amount. Step one is to request a pension forecast from PPPA with the max amount available for commutation. Transfer that figure onto the commutation application form and Bob's your uncle.
I passed this advice on to several friends in the same boat, and all got their cash on time, so the moral of this little dit is play by the rules when it is something that really matters.