AndreaC wrote:

I'm about to start a new job and this will involve travelling from
Hounslow to Warren Street.

If I use the train it's a simple case of going to Waterloo and then up
on the Northern line, but I'll also have the option of using the tube.
The most obvious route there seems to be Piccadilly line to Green Park,
change to the Victoria line there and go on to Warren Street.

However, I seem to recall that once, a long time ago, in the vague
mists of time I tried to change or possibly even exit at Green Park and
it was a flipping horrible station to navigate.

Yes, there are long walks between the lines there so it takes quite a
long time to change. Never change/enter/exit tube at Green Park unless
you have to for some really good reason.

The alternative as I see it is to stay with the Piccadilly line to
Leicester Sq and then change to the Northern line there.

Very much better, the best choice in my opinion and probably the
fastest. Even though Leicester Square can be quite messy when crowded
too it is still much easier and straight-forward to navigate.

Any suggestions or advice from those of you who know these things
better than me? Any idea of journey times?

The TfL Journey Planner says about 50 minutes.
http://journeyplanner.tfl.gov.uk

--
Olof Lagerkvist
ICQ: 724451
Web: http://here.is/olof