HELP roof lantern and upstand constant leak

[andypeters]

My builder ordered the wrong size lantern this was 18 months ago and now has gone into liquidation , I have had nothing but issues ever since with leaks. We have had numerous roofers round trying to fix it but none successful as water keeps getting in.
Obviously upstand is far too big and roofer has used mastic and sealant all around to fix it but to no avail. As you can see the workmanship is terrible but just don't know what else can try and do to fix it. Any help would be appreciated.

 

[freddiemercurystwin]

The first roofer in should have made the opening smaller instead of skirting around the issue, it probably needs to all come apart to really see what's what.