Andrew Gabriel wrote:

Also, quite apart from the earthing problems with long cables,

But in the lawnmower context, (a) the mower will almost certainly be a
Class 2 (double insulated) appliance so it won't rely on earthing, and
its metal frame won't rise in voltage relative to the ground during a an
earth fault should you mow through the 3-core cable: (b) the extension
lead should be plugged into an RCD-protected socket, so the earth fault
loop impedance issue becomes largely irrelevant, provided that the earth
isn't completely o/c. If the house electrical installation is old and
doesn't have RCD-protected sockets then use an plug-in RCD adaptor, as
Andrew said.

you might be dropping a significant proportion of the mains
voltage across the cable when the mower is running. Most electric
mowers use cheap universal motors which won't mind this too much,
but better quality mowers use induction motors, which don't like
running at reduced voltage on-load (could overheat).

We don't know what cable size was used - but 30 m of 1.0 mm^2 flex will
only be just over 1 ohm and I can't see that causing too much in the way
of problems. In 0.75 mm^2 though things will be getting rather marginal.

--
Andy