UIWebView isn't accepting multiple touches

To display attachments on a messaging system I'm developing for my app, I've got a UIWebView which is presented in a ModalViewController, displaying the file contained in the attachment.

If it's any help, the code I'm using to display the file is as follows:

NSUrl docUrl = NSUrl.FromFilename(filePath);
NSUrlRequest request = new NSUrlRequest(docUrl);
webView.LoadRequest(request);

(filePath is a string of the Path to the file).

This works fantastic for displaying PDFs, text documents etc.

However, the UIWebView isn't accepting any multi touch gestures (pinch to zoom etc) - I've set

webView.MultipleTouchEnabled = true;
webView.UserInteractionEnabled = true;

yet still no luck. Will I have to use a GestureRecognizer or am I missing something obvious?

I'm aware I'm using Monotouch but Objective C answers are more than welcome : )

Is it possible that scalesPageToFit is set to NO? That might cause similar behavior to what you are seeing.