ok, so it appears I may have more joy using this:
$AllocatedIPs = $extnet.ExtensionData.configuration.ipscopes.ipscope.allocatedipaddresses.ipaddress
If I do a count on this it currently matches the 157 allocated IP's. So I will update my script and then feedback regarding whether this issue has bene resolved.
I can confirm this has resolved the issue I was having with manual IP assignments.