Robin, whats your problem?
No need to come over all 'anti', are we not on the same side?
All I said was that I HAVE on several occasions lengthened hulls using styrene sheet as a mould medium, I know full well that its not ideal, but I've done it & it works, of course it comes up 'horrible' , but 'horrible' can be fixed.
Yes, polythene is better, but we don't all have access to it, thin ply, alloy sheet etc. can also be used, but polystyrene does as well. In other words, your particular method isn't necessarily the ONLY way of coming to the same end.
Remember that this isn't a kit that is being 'correctly' (factory) converted, but simply one way of a modeler altering his/her model & thus exploiting his/her skills, Isn't this what its all about???
Lighten up!
Lawrie