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Correction:-

"....As part of this exercise,  a check was made of the link between the two 
coordinate systems which depended on the survey made (I think in the late 
nineteenth century {someone will be able to correct me on this no doubt} ..."

I'll correct myself:  I meant of course late _eighteenth_ century as the Airey 
instrument was established in the late nineteenth.

The power of the internet is amazing,  here is the information about the 
Greenwich meridian(s), in all it's glory at:-

http://gpsinformation.net/main/greenwich.htm

And a relevant quote from the above link :-

"Even today, it can be confusing as there are four Meridians all passing 
through the Old Royal Observatory.

The earliest is Flamsteed's, named after the first Astronomer Royal, which was
established in 1675. In 1725, Edmund Halley, the second Astronomer Royal
established a second Meridian.

The third was defined by another Astronomer Royal, James Bradley, in the mid
18th century, and is still used as the basis for map-making in Britain today.

The fourth Meridian was established in 1851 by yet another Astronomer Royal, 
Sir  George Airy, who set up new measuring equipment in a room alongside 
Bradley's  original equipment. It is the positioning of this neighbouring 
equipment, just 5.79  metres (19ft) away, which eventually became the basis 
for international time. Due to the convergence of meridians of longitude 
towards the poles, Bradley's Meridian is  5.9m west of Airy's where they 
cross the South Coast of England, and 5.5m west  where they cross the East 
Coast.

As the pace of development and travel accelerated in the 19th century, it became
clear there would have to be a common, world-wide standard for timekeeping. In
1884, 25 countries reached agreement at a conference in Washington, USA, that
Airy's Greenwich Meridian would be adopted as the `Prime Meridian' - zero
degrees - from which time could be set and from which other points of longitude
could be calculated. Over a period of many years, countries which had not
necessarily been party to this original agreement accepted and adopted the
decision.

So since 1884, the Airy line has been The Greenwich Meridian, although for
practical mapping purposes in Britain (excepting hydrographic charts) the 
Bradley line continues in use as the zero meridian. The difference between 
the two is known and well defined - and is important scientifically - but for 
most day-to-day purposes has no real consequence."

-----------------------
Douglas Denny.
Chichester England. 


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