verbena wrote:

In the same vein, just before J8 on the Londonbound side of the M4,
there's a curious bit where the roadside fence stops and it looks
like there was some sort of exit at some point. Google Earth just
shows a field beyond and a fairly new housing estate. It does
however look as
if something turned away from the motorway at some point in the
past.
Was it a contractors depot or similar?

I think you're referring to the original J8 which was on the London side
of the current J8/9.

When the Maidenhead by-pass section of the M4 was constructed, there was
a junction where the M4 crosses A308. The slip roads were on the London
side of the A308 bridge, and led to T-junctions with A308, uncontrolled
as far as I remember. Going west from here, the motorway then curved to
the north-west and headed for Maidenhead Thicket with the intention of
continuing westwards to the north of Reading.

When it was decided to route the M4 south of Reading a new junction was
needed between the Maidenhead Thicket route (now A404(M)) and the new
alignment. When the junctions were numbered, the A308 junction became
J8, the new junction was to be J9, and the two junctions on the Thicket
route became J9a and J9b. However, it was then realised that J8 needed
upgrading and was very close to the planned J9. The upgrading was
achieved by building the A308(M) spur to a combined junction J8/9, and
the old J8 was closed.

You can still see the original curve of the M4 just to the north-east of
the J8/9 roundabout.
--
Richard J.
(to e-mail me, swap uk and yon in address)