Why not answer your own question by going back to basic physics that you should have covered in 4th or 5th form.
Assume your trimmer is rated at 500 Watts. At 240 Volts, that's 500/240 = about 2.08 Amps.
What voltage drop will be caused by the cable?
Let's assume you've chosen a weedy mains extension cable and that it has a conductor of - say - 0.75 sq mm
A quick Internet search suggests a resistance for 0.75 sq mm copper cable of 26 Ohms per 1000 m ( FYI, a 0.5 sq mm cable has a resistance of 40 Ohms per 1000m)
Your cable proposed is 58m, which is two conductors of 58m, or a total length of 116m. Call it 100 m.
Resistance is therefore 116 * 26/1000 = 3.01 Ohms
Voltage drop is current X resistance, so 2.08 A X 3.01 Ohms gives a voltage drop of 6.26 Volts. Scarcely worth getting out of bed for.
Power dissipation in the cable is Volts X Amps (or Current squared X resistance). So 2.08 A X 6.26 Volts = 13.02 Watts.
Think how warm a 15 Watt light bulb can get and you'll understand why makers of extension leads warn you not to leave them tightly coiled. Unwrapping the cable gives the heat a chance to dissipate.
If your trimmer is rated at 750 watts, just increase the volt drop and power dissipation by 50% (Or do the sums for fun)
Did that help?