"Brownfingers" wrote in message
...
our garden slopes down to a park which is beyond a common access
track....

Is there more sloping ground above the garden? If so, the answer is to build
an intercepting drain (which could be a gravel filled ditch under the grass)
at the highest part of the garden and to pipe the water from that to the
bottom. The garden then only has to deal with the water falling on it, not
that plus any water falling on the hill above.

Colin Bignell