Man of Honour
It's not going to be an earth fault (as mentioned above) as that mower is Class II equipment so it has no earth path and it's unlikely to be inrush current either as that would cause overload, this sounds like leakage.
Would be handy to see pics of both fuseboxes (and what exactly is tripping their side) & the earth terminals/incoming service, I'm guessing by the age of the property the earthing system is TN-C-S so that might have something to do with it although how you're drawing power upstream of their RCD on either the L or N I don't know.
Their RCD might just be close to tripping anyway, they're designed to trip at 30ma but in practice they're around 25ma, so they could already be leaking 23-24ma and somehow you're just tipping it over, a decent sparky can ramp test RCDs to find what they trip at by the way & how much leakage is already going on.
I wouldn't bother with speaking to the manufacturer, if the mower had a genuine fault your RCD would trip too
Edit: This is all assuming you have RCDs of course, they weren't a legal requirement for new installs until 2008 so the builders might not have bothered with them.
Would be handy to see pics of both fuseboxes (and what exactly is tripping their side) & the earth terminals/incoming service, I'm guessing by the age of the property the earthing system is TN-C-S so that might have something to do with it although how you're drawing power upstream of their RCD on either the L or N I don't know.
Their RCD might just be close to tripping anyway, they're designed to trip at 30ma but in practice they're around 25ma, so they could already be leaking 23-24ma and somehow you're just tipping it over, a decent sparky can ramp test RCDs to find what they trip at by the way & how much leakage is already going on.
I wouldn't bother with speaking to the manufacturer, if the mower had a genuine fault your RCD would trip too

Edit: This is all assuming you have RCDs of course, they weren't a legal requirement for new installs until 2008 so the builders might not have bothered with them.
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so the only time I use one is when I fix the cable on a neighbours one