Apologies for all the abbreviations in the title 
I have 2 DC from a previous relationship, aged 12 & 13. I split from their father 8 years ago and moved some distance away to escape DV at his hands. He has had infrequent contact with them since, telephone contact most weeks but only seeing them once a year if that for an afternoon. He is on the birth certificate but doesn't have PR as they were born before 2003 and we weren't married. I have been with DP for 5.5 years, living together for 2 and have an 18 month old son together. DP and I are not married.
We (DC included) would like DP to have PR for the DC for practical reasons such as holidays abroad if one of us needs to return separately unexpectedly, granting permission for medical treatment if I am unavailable etc.
XP has recently started a court case for contact, despite being offered far more than he ever took during the last 8 years, and has shown during the case so far that he would be very against my DP having PR and would fight it.
My question is, how difficult could he make it if I applied for DP to have PR of the children? I have no doubt that he would make it as difficult as possible, but I don't know just how difficult that is IYSWIM? Another question is would he get legal aid to oppose the application? Sadly for the DC, if there is a chance he will be out of pocket, he simply wouldn't bother which is of course good news for getting the PR sorted, if not sad for the children that their Dad isn't interested enough to put his hand in his pocket for something he believes in, just to stir trouble if it doesn't cost him anything.