Just got back from our Jersey trip to find the following from Cormaic,
asking me to post it on his behalf. Hope it's not too late to be useful:

Rust is a nightmare stain on paving, and one of the hardest to
shift. This is why it's never a good idea to cast mosskiller over
paving: the ferrous content creates a near-permanent stain.

Some flags are more badly affected than others. Some concrete
flags are seemingly immune while others can't be cleaned at all - it's
all to do with the type of concrete and the way in which the flags were
made. Much of the imported Indian sandstone and limestone is also badly
affected. The key to treatment is to try cleaning a small, discreet area
FIRST, and allow the treatment to "settle" for at least 48 hours before
spolshing it all over the rest of the paving - sadly, some cleaners have
a slow-acting adverse reaction with flags, and it can take a day or two
for this to be realised.

There's some generic advice on my website...

General rust stains...
http://www.pavingexpert.com/stains.htm#rust


Rust from moss/weedkillers...
http://www.pavingexpert.com/maintain_02.htm#moss

--
Sacha
http://www.hillhousenursery.co.uk
South Devon
(remove weeds from address)
'We do not inherit the earth from our ancestors, we borrow it from our
children.'