I've finally been investigating the wonderful world of customising web listener forms on TMG - and after a bit of hunting found this great article online:

http://richardkok.wordpress.com/2010/10/22/creating-a-custom-forefront-tmg-2010-owa-fba-logon-page/

which got me going pretty much straight away- the only caveats I'd like to highlight to anyone out there is the rather important cacheing remark he notes just at the bottom of the article- for some reason, even if a relatively underused, two server TMG cluster, is that the changes are far from instant, and require a restart of the TMG services to show the changes- not ideal when you're tweaking and testing different colour schemes- goodness only knows why the TMG's bother to cache this content!