There are many things to try to deter cats from your gardens and area, if you live in the UK not sure what you have access to...Repellents and devices designed to startle the cat "in the act" will work best to condition him to avoid the area. Never use poisons.Most commercial repellents are based on the simple mothball compound. Mothballs or moth crystals, encased in cheesecloth bags to help protect the soil, work quite well to keep cats from digging in garden areas or potted plants. May have to reapply due to weather conditions.
For areas where cats want to dig, ornamental pebbles may be an effective deterrent. Avoid those that are very round or smooth, as they make a great cat bed. Rocks or pebbles should be firmly secured into the dirt or mulched area so they can't easily be moved or overturned. Small-gauge chicken wire can also be buried under a light layer of dirt or mulch, and may even restrain some weeds. The sharp pebbles and/or rough wire will be uncomfortable to soft paws. Scatter dry beans, macaroni or birdseed on a metal tray (disposable pie pans or cookie sheets work great and are inexpensive). Balance several trays along the fence, porch or deck railing, the windowsill, or around the edge of any vehicle where the cat jumps onto the surface.
When the temperature permits, turn on a water sprinkler during the usual time of disturbance (dawn or dusk if the cat is on your property to hunt). A timing device for the sprinkler, set to a staggered schedule, will help discourage those intelligent cats who would otherwise simply avoid the area at "regularly wet" times of day. As a variation on the "falling tray" method, set shallow plastic lids filled with water on each end of the tray to add a shower to the noise and movement of the falling tray.
Hope this helps....Sorry it's so long but tried to give you alternatives.