This is an age old problem that most of us face most of the time and there are hundreds of different products and methods available all with different levels of effectiveness.
The first thing I can advise is that gravel is a definite no no! Any gravel area in a garden invaded by cats will usually just become one giant cat tray, larger cobbles and pebbles will usually be ok just avoid finer aggregates.
Silent Roar (Lion Poops)
The most popular product I have come across is called ‘silent roar’. Now this is going to sound a bit mad but they are basically pellets soaked in lion poops essence. I have just checked their website and due to EU law they have to sell the product as a fertiliser and not a cat repellent. This has something to do with the fact that they cannot control the exact make up of the product due to the fact that every lion poops is different depending on what they eat.
But anyway, it really does work – cats are territorial and what cat is going to be brave enough to enter the territory of a much larger lion?! It costs about 9£ a box so is fairly cheap and also harmless.
The Catwatch Ultrasonic Cat Deterrent
There is another product called the catwatch ultrasonic cat deterrent that has an infrared sensor that when tripped emits a high pitched noise (inaudible to us) that scares the cat off. It costs around 50£ and is proven to be really effective.
It is also accredited by the RSPB who recommend it as being effective. You may need a few of them to cover the garden and also they are more effective when plugged into the mains as opposed to running on batteries so you would need to buy the mains adapters and possibly run cables into the garden which means it is not the cheapest method.
There are many other alternatives such as spreading ground chilli powder to lemon peel around the garden, but these are really short term solutions seeing as it would need to be repeated regularly to remain effective. So I would try the lion lion poops!