Sorry for not posting a reply sooner, I've been kept very busy lately. As soon as I get a couple of hours free I'll zip up a sample project and post it here. Probably over this weekend.
Here's the plugin for version NOP V 1.8 http://www.wannaskim.com/Nop.Payment.SagePayDirect.rar Originally made by Miles and Modified by me. This is provided AS IS and with no warranty. IT worked for what I needed at the time.
Supports capture, refund, partial refund, void, 3dSecure etc.
I should have a version of this working for v2.8 within the next month and I'll get it uploaded to codeplex.