If you send it in now, you'll be grand. Honestly, there was no hope for me realising because I thought the lawyer had taken care of everything (she paid all the fees up front for us). She has since moved to a different firm. The first we heard was when we got a letter from our new lawyer saying it hadn't been paid. The most frustrating thing is that we don't know if this is a mistake (others in the thread have paid it, only to be asked for it again later on), or if she really didn't pay it. Grrr!
However, lucky for you, I did learn in my frantic 'does an RPRF mean we're getting close?!' googling that it is "best" to send it either when you apply or 'anytime before they ask for it.' So, yes, you can TOTALLY send it now and everything will be fine. Just pay it online, print off the receipt, enter the payer's details and Client ID Number and attach a note that explains you're paying it now because you didn't realise you could pay prior to the request (that's if you're worried it will get lost/they won't know what to do with it when they get it). Worst case scenario, London will send the RPRF request when the time comes and you can reply directly to the VO with an email and attachment showing it's already been paid. It seems others have done so when they received an RPRF in error and London has gotten back to them fairly quickly about it!