Not having Sycamore trees in ouir part of the globe, I can only speak of
my experience with killing our invasive and spreading Poplars. Here is
what worked for me and my neighbours.
1-drill holes, at 3" spacing between them...and as deep as your drill
bit will allow you.
2-fill each hole with a mixture of 1/2 Herbaside + 1/2 crude oil
3-cover stumps with plastic/cello whatever, just keep the area from
evaporating.
4- continue to keep holes filled through one complete season......not
necessary to do during winter months however. This WILL kill the
remaining root system......
Ldymac
Tony wrote:

Please advise,
i recently cut down a sycamore tree that was starting to get too large and
was too close to buildings in the garden.
i have been told that because of its size, rather than try to dig and chop
out the roots, i should drill the stump and inject it with ??
does anybody know what i should use to kill off the stump and roots?
thanks in advance