Someone provided a solution, via the Craft Google community, but it was suggested that I also post it here for others to use.
<nav>
<ul>
<li><a href="{{ siteUrl }}" {% if craft.request.firstSegment == 'homepage' %} class="active" {% endif %}>Home</a></li>
<li><a href="{{ url('work') }}" {% if craft.request.firstSegment == 'work' %} class="active" {% endif %}>Work</a></li>
<li><a href="{{ url('about') }}" {% if craft.request.firstSegment == 'about' %} class="active" {% endif %}>About</a></li>
</ul>
</nav>
The above was adapted from the demo site install (I think) and although 'work' and 'about' work fine 'homepage' did not. I used 'homepage' as this is the 'handle' in the craft sections part of the cms so I thought that would be correct.
I initially found this How can I add a dynamic "active" CSS class to the navigation on any given page? but not being that dev focused found the many alternatives a bit confusing.
It was suggested that I add a 'section' to the homepage template and so I added {% set section = "home" %}
after {% extends "_layout" %}
in the homepage index.html.
I then amended my first <li>
from above to be just
<li><a href="{{ siteUrl }}" {% if section == 'home' %} class="active" {% endif %}>Home</a></li>
which worked as required.
I hope that helps.
If anyone can explain why my original doesn't work, in layman's terms that would be much appreciated.
Thanks
Laurence