Always useful to have one when going detecting.
Usually a spade but a fork can be useful in a ploughed field if the ground is hard or stony. Metal detecting shops sell special spades though a normal garden spade with the blade cut to a point is used by many. Choose one with a thickened top edge where you put your foot. Repeated digging with a spade with a thin edge will tend to ruin the soles of your boots resulting in a wet foot when you tread in a puddle.
Good headphones help block outside noise and make faint signals easier to hear. Some have built in volume controls which gives water a way inside. Some detectors have a 1/4 inch jack socket and others have 3.5mm.
Real metal detectorists go out regardless of the weather and need suitable clothing. Wellington boots, waterproof coat and trousers if it is raining. Don't tuck the trousers into the boots, the rain will run down them and fill your boots. Don't wear boots with steel toecaps or metal rings for the laces. You want to find coins not your own toes. Always wear gloves. A pair of disposable gloves to keep your hands clean with thin gardening gloves over the top for protection. Latex disposable gloves are better in the winter as they provide a good thermal layer. Vinyl disposable gloves are better in the summer as your hands will sweat less. Some people can have an allergy to latex though.
This allows you to support your detector from a harness with an elasticated chord and take much of the weight off your arm. This is desirable with heavier detectors or for people with conditions such as tennis elbow. It gives the added advantage of being able to let go of your detector and dig without it dropping in the mud.
Basically a small metal detector for locating a find in the hole. They normally only have a range of a few CMs. Better models will change tone the closer they are to the target.
Useful to record find spots. Accuracy is dependent on the number of satellites in view and will be less accurate in woods. Handheld GPs will tend to give an accuracy in 10s of metres so will not get you back to an exact spot later. Differential GPS systems are used by professional surveyors but they will be beyond the budget of most detectorists. Modern smartphones normally have GPS built in but rely on a good phone signal to aid the GPS and tend not to be waterproof or mud proof.
Bag or rucksack
Somewhere to keep your sandwiches, thermos, spare batteries, car keys, etc. Also useful to carry your hoard of plough shares back to the car. Preferably choose one which is waterproof.
Usually worn on a belt or tied round the waist. These are useful to put finds and rubbish in. It is useful to also carry a small plastic container containing some cloth to hold and protect better finds.