Is this - via A40/M40 - going to be a nightmare ?

Any idea on time ?

Leaving Harrow in the morning, returning evening.

You will be going against the prevailing traffic, so it shouldn't be a
problem once you get to the A40.

...although sometimes Handy Cross (M40 junction 4) and the A404 can be a
bit of a pain.

Exit at J3 and use the Flackwell Heath / Sheepridge Lane route then.

That might be a good idea going to Marlow in the morning (not too familiar
with the traffic westbound coming off at J4 in the morning), but in the
evening, coming through Marlow Bottom, to go straight up to Handy Cross and
onto the M40 shouldn't be too much trouble at all - the roundabout itself is
never that bad from what I've seen, it's just the queues up to it, mainly on
the A404 from the South.

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