I get this error when I submit a form:

[Recoverable Error] Object of class SS_HTTPRequest could not be converted to string.

Seems to be this code causing it. Removing this code (for testing, just two fields) allows the form to submit:
if($bookingid && $member) {
$booking = DataObject::get_one('VehiclePageBookings',array(
'ID' => $bookingid,
'MemberID' => $member->ID
));
}

There are no errors when the form fields get loaded. Loads the correct data from the above code. Just throws a error when you press submit.

Thanks