My views are broken for integer fields (broken/missing handler)

Recently any views I have configured which either show integer fields or contain them in filters, show them as 'broken/missing handler or Array' in the edit view screen. Similarly if I try to add an integer field of a
content type to a view (either for display or as part of the filter) it is not shown in the list of available fields for that content type. Any non-integer fields are shown however.

I'm a bit stuck on how to fix this (without wiping my database/Drupal installation and starting again), so any ideas would be very welcome!

The only thing I can think of that may have caused this, was some code I added to dynamically change the SQL produced by the filter. I added code to the end of views.module (temporarily until I've worked out a safer place):

function views_views_pre_execute(&$view) {
drupal_set_message($view->name);
if($view->name=="PRODUCT_LIST") {
$view->build_info['query']=str_replace("field_price_min_value =","field_price_min_value >=",$view->build_info['query']);
$view->build_info['query']=str_replace("field_price_max_value =","field_price_max_value >=",$view->build_info['query']);
}
}

However even with this code removed, the broken/missing handler error still shows.

Thanks in advance for any help with this,

James