I have an urgent need to decrease to ~7ft a line of Leylandii along one
edge of my garden. Using a handsaw is good for me but takes too long. I
am not prepared to spend money on a chainsaw for this single use. What
would be the issues in using an angle-grinder? The sap is obviously
relatively viscous and non-seepy at the moment so I wouldn't expect
catching from that to be a problem. The maximum trunk diameter is
probably 3". What do folks think?

Three inches -- pah!? A new bow saw blade will be through that in 30
seconds, and with an assistant pulling the trunk away from the saw cut
witha rope you won't even need to cut a "kerf" out first to stop it
binding. Please, please don't hire a chainsaw unless you've been trained
how to use it, even to drop little leylandii. Especially don't use one
up a ladder...