I think I'd talk to the landlord and ask her to sort it out, otherwise you'll be forced to go to the council. If she's faced with the prospect of a court appearance/fine, she'll find some way of getting in there to get the work done. As long as you're paying your rent on time and are not in arrears or anything, you have the right to enjoy your property, including the garden. I was involved in a dispute years ago when I was a student, as the upstairs flat was full of dole-dossers (not stereotyping, but they were up all night partying and slept all day, apart from signing-on day). It meant that I was getting behind on coursework, as I couldn't sleep (had the council noise monitoring equipment in). As soon as the landlord realised that I had the council involved and he was likely to be in court for breach of contract (the expectation of quiet between 11pm and 7 am is implicit in most rental contracts), he served an eviction notice on the upstairs. When he actually went in to clear it out, the police got involved as he found loads of used needles & other bits & pieces, it looked like the chap was pimping the 2 girls he lived with for drugs and money for drugs. I've never been so glad I shared the flat my best mate and her bloke, I dread to think what could have happened if someone got the wrong flat and there were only us two girls there!
Most landlords just want a quiet life, so they'll normally sort things out before the council gets involved. Mine just needed a kick up the backside, as he was always saying he'd sort it and never did, he thought I was making it up about the noise and partying, as soon as he found out about the noise monitoring equipment the council had put in, he moved like greased lightning to get the upstairs flat emptied and let out to a working couple!