3 acres at £25/acre is only £75, which doesn't buy very much nowadays. Particularly when you take into account the time taken to set up his machine, drive to/from you, pay for the fuel and maintain his kit. If he has to raise an invoice as well then it will take him even longer.
Personally I would rip his arm off if he was available to do my fields!
To buy your own kit:
Quad, say £4k
Flail, say 3k
Fuel, £??
Maintenance, £??
So he could top your fields 106 times for your outlay of machinery cost, ignoring your time, fuel and maintenance costs. If you topped your fields twice a year say, they that is over 50 years worth of topping before you have covered your outlay! If you want to top your fields every other week, it would be a different story of course.