hiring or buying equipment

I'd honestly weigh up the costs. The merry tillers, new, cost quite a bit. Hire shops seem to charge far too much for hiring out stuff and usually if you're going to use it 3-5 times, you would have been better off buying one outright. If your allotment shop has one and it's quite cheap like a few quid an hour, I'd be more tempted to just hire from the allotment shop. They're usually locked up more securely than any sheds in the allotment and you don't have too far to go to get it unlike from a hire shop. And while you're hiring for the cheap rate, you can get some work done while checking out the buy&sell papers or freecycle for a bargain merry tiller like John picked up  

We've dug over almost all of our beds by hand but it's taken us many months and we have heavy clay soil too.