Greetings! I have, with the very kind assistance of someone on Stack Overflow, sorted this, so am posting the solution here. I have since learned that people like that it's disclosed if a question is cross-posted here and at SO, so apologies that I didn't do this - I know for future questions. I first set the lang attribute to lazy:

use FindBin qw( $Bin );
use TopTable::Maketext;

has "lang" => (
  is => "ro",
  isa => "TopTable::Maketext",
  builder => "_set_maketext",
  lazy => 1,
);

sub _set_maketext {
  my ( $self ) = @_;
  my $class = $self->class;
  my $app = $self->_app;
  my (@locales, %inheritance);
  my $config = $app->config->{I18N}{locales};
  $app->log->debug( sprintf( "app: %s, class: %s", $app, $class ) );
  printf( "app: %s, class: %s", $app, $class );
  
  foreach my $locale (keys %$config) {
    push(@locales, $locale);
    $inheritance{$locale} = $config->{$locale}{inherits} if defined $c
+onfig->{$locale}{inherits};
  }

  my $dir = Path::Class::Dir->new( "$Bin/..", "root", "locale" );
  TopTable::Maketext->load_lexicon(
    locales => \@locales,
    directories => [$dir],
    gettext_style => 1,
    inheritance => \%inheritance,
  );
  
  return TopTable::Maketext->get_handle( "en_GB" );
}
[download]

This worked, but produced the message Lexicon has already been loaded for TopTable::Maketext, which suggested Catalyst loading the lexicon was a global action, so actually I was able to get the _set_maketext method down to:

sub _set_maketext {
  return TopTable::Maketext->get_handle( "en_GB" );
}
[download]

So far so good, but I then had to work out how to get the user's locale into the call to get_handle(). I have managed to get this working using an ACCEPT_CONTEXT sub in the model (note the if ref( $c ) eq "TopTable" check - as the comment says, the model seems to get called by Catalyst as part of the instantiation, before my code kicks in, and in these cases, $c is the string "TopTable", not a ref to the TopTable object so we can't call ->locale on it):

package TopTable::Model::DB;

use strict;
use base 'Catalyst::Model::DBIC::Schema';

__PACKAGE__->config(
    schema_class => 'TopTable::Schema',
    
    connect_info => {
        dsn => 'dbi:mysql:toptable',
        user => '',
        password => '',
    }
);

use TopTable::Maketext;

sub ACCEPT_CONTEXT {
  my ( $self, $c ) = @_;
  
  # We have to check the ref of $c here because this model seems to ge
+t called by Catalyst as part of the instantiation, before my code kic
+ks in
  # and in these cases, $c is the string "TopTable", not a ref to the 
+TopTable object.
  $self->schema->_set_maketext( TopTable::Maketext->get_handle( $c->lo
+cale ) ) if ref( $c ) eq "TopTable";
  return $self;
}

1;
[download]

I've changed the schema thus:

use TopTable::Maketext;

has "lang" => (
  is => "ro",
  isa => "TopTable::Maketext",
  writer => "_set_maketext",
);
[download]

This is now working as expected, hurrah!