Still trying to remove the casing

I've used cardboard as suggested. I've also located the exploded diagram
on the Hornby web site. (This is all related to a class 121 Bubble Car).
But the casing still wont separate from the chassis.
The exploded view seems to show 6 locating lugs and I think I've got them
all disengaged.
What Hornby don't tell you is that the exhaust pipes also unclip from the
body (I've discovered how to do that). But I'm still struggling. The
issue now seems to be the interface (if I can call it that) between the
buffer beam and the chassis. That seems very tight and I don't have a tool
that is narrow enough to get into it easily. I'm beginning to feel that
brute force (applied gently and progressively) may be the answer but if
anyone has some further suggestions I would be grateful.