I've got the same problem - attacked the Xant's accumulator sphere on Wednesday night and put teeth marks in my new Boa tool!
It's looking very sad - they just aint that tough.
Hammer and chisel method tomorrow, me thinks.
2000 Clio 1.2 8-valve
Couple of Merc 190s
2009 petrol Focus
1998 Xsara TD, 293K - gone but not forgotten.
205s, 405, ZXs all loved and crushed.