[enuddleyarbl]

With these classes:

h1 + p, h2 + p, h3 + p, hr + p {
  text-indent: 0;
}
h1 + p::first-letter, h2 + p::first-letter, h3 + p::first-letter, hr + p::first-letter	{
  font-weight: bold;
}

.noindent {
  text-indent: 0;
}
.firstletter::first-letter {
  font-weight: bold;
}

That seems to handle it. The two hn + p classes pick up most situations and <p class="noindent firstletter"> picks up any troublesome cases.