Can CiviCRM be integrated into a bespoke custom built cms, I see that you can download it for drupal, wordpress and joomla but our cms was built specifically for our non for profit, if someone could help me with this that would be fantastic
It's possible that you could build an integration component to make it work with your custom cms, but it probably wouldn't be worth the effort.
I would say it is possible but you would be in not well-charted territory. I'm assuming you are a developer. You'd need have a thorough understanding of how the CMS works under the hood.
I am not certain if CiviDesk is maintaining a "standalone" (CMS-free) fork.
Along the lines of the other the other replies... you'd probably be better off adding single-sign-on features to your CMS for a seamless integration with a minimal drupal instance that houses Civi.