Fuel expansion tanks - trouble refitting

[aec]

I'm trying to refit the off-side expansion tank, but whatever I try I can't get it back in.  I read a post about pushing the tab over the bolt first, then pushing the back of the tank until it clicks into place but I'm pushing as hard as I dare on the bottom of the tank with a bit of wood and it won't lock in. Similarly, I can locate the "lump" on the tank in the depression nicely but the lug doesn't line up to the upper bolt despite brute force pushing it.

Am I doing something wrong here?  It came out reasonably easily and everything is all clean now so I wasn't expecting it to be so difficult to replace I don't want to leave it hanging from the bolt with being located properly.

All I can think is that the plastic is a lot stiffer than when it was new preventing it from flexing into place?  if that's the case I think I'll have to try drastically enlarging the mounting hole. 

[Mocki]

Try pouring boiling water over it ?

[aec]

Good idea, worth a try