Ok, found it.
It is the anchor towrds the bottom of "/modules/Fast_Lane_Checkout/checkout_0_enter.tpl". It has been self-closed, but anchors should not be self-closed, as far as I'm aware.
Change:
<a name="regdlg" />
to
<a name="regdlg"> </a>
as I've also read somewhere that IE7 doesn't like empty anchors.
Job done.