1. Change in situation is grounds for a change to the separation agreement - but your attorney would have told you this. 2. In the UK (and in the EU I think) what is established during the separation becomes the status quo and in the event of a divorce, courts try and maintain the status quo because any further change is viewed as disruption to the children. What this means for you is if you only see the kids every 2nd weekend and this has been the norm throughout the separation, then the court views it as the status quo and it is harder to change. If you pay your W x amount each month, then the court views that as the norm and will try and maintain it.
If there is anything you want to change in respect of the separation then do it now because the longer it goes on then the longer the 'norm' is established and the more difficult it will be to change.