The problem with the WebView content not responding to any clicks, scrolls, etc appears to have been caused by an unused CollectionView in the source.
I had this
<Grid Margin="0"
Padding="{StaticResource SpacingDefaultTop}"
BackgroundColor="{StaticResource SecondaryColor}"
IsVisible="{Binding TrustPayFrameVisible}">
<WebView x:Name="PaymentWebView"
Scale="1"
Source="{Binding WebViewContent}" />
<CollectionView x:Name="ItemsCollectionView" ItemsSource="{Binding Items}">
<CollectionView.ItemTemplate>
<DataTemplate x:DataType="api:UserCard">
<Grid>
<Grid.RowDefinitions>
<RowDefinition Height="Auto" />
</Grid.RowDefinitions>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="100" />
<ColumnDefinition Width="6*" />
<ColumnDefinition Width="Auto" />
</Grid.ColumnDefinitions>
<Label Grid.Column="0"
FontSize="{StaticResource Body1FontSize}"
LineBreakMode="TailTruncation"
Text="{Binding CardType}" />
<Label Grid.Column="1"
FontSize="{StaticResource Body1FontSize}"
LineBreakMode="TailTruncation"
Text="{Binding CardLastFourDigits}" />
<Label Grid.Column="2"
FontAttributes="Bold"
FontSize="{StaticResource Body1FontSize}"
HorizontalTextAlignment="End">
<Label.FormattedText>
<FormattedString>
<Span Text="{Binding ExpiryMonth}" />
<Span Text="/" />
<Span Text="{Binding ExpiryYear}" />
</FormattedString>
</Label.FormattedText>
</Label>
</Grid>
</DataTemplate>
</CollectionView.ItemTemplate>
</CollectionView>
</Grid>
The CollectionView is no longer required, but I just hadn't removed it. When I did, the standard WebView then worked correctly.
<Grid Margin="0"
Padding="{StaticResource SpacingDefaultTop}"
BackgroundColor="{StaticResource SecondaryColor}"
IsVisible="{Binding TrustPayFrameVisible}">
<WebView x:Name="PaymentWebView"
Scale="1"
Source="{Binding WebViewContent}" />
</Grid>