Dear Andy_S,

You can use Cache Dependency for your problem.

ASP.NET allows you to define the validity of a cached item, based on an
external file, a directory, or another cached item.

These are called file dependencies and key dependencies.

If a dependency changes, the cached item is invalidated and removed from
the Cache. You can use this technique to remove items from the Cache when
their data source changes. For example, if you write an application that
processes financial data from an XML file and renders it in a graph, you can
insert the data from the file in the Cache and maintain a dependency on that
XML file. When the file is updated, the item is removed from the cache, your
application rereads the file, and a new version of the item is inserted.

Note   The Cache has no information about the content of the items it
contains. It merely holds a reference to those objects. It also provides
methods to track their dependencies and set expiration policies.

go through these links

INFO: ASP.NET Caching Overview
=====================
http://support.microsoft.com/kb/307225/EN-US/

Caching Application Data
===============
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/cpguide/html/cp
concacheapis.asp

CacheDependency Class
==============
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/cpref/html/frlr
fSystemWebCachingCacheDependencyClassTopic.asp

CacheDependency Members
=================
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/cpref/html/frlr
fsystemwebcachingcachedependencymemberstopic.asp

bye
Venkat_KL