Well the bigger the better. 50 US Gallons would be the minimum when doing an African Cichlid tank. Remember, These fish grow to an average of 6" and grow to 8". When planning your tank, First check your water parameters (pH, Ammonia, Nitrite, Nitrate) then plan your fish around it. There are three major lakes in Africa (Lake Malawi, Lake Victoria, And Lake Tanganyika) Each of them have slightly different water parameters.
Lake Malawi is home to most of the popular aquarium Cichlids. labidochromis caeruleus
(Electric Yellow Cichlid) is one of the more popular non aggressive Cichlids. Pseudotropheus Acei Is also a more non aggressive Cichlid. When adding them, Add the least aggressive species first as to let them get a territory and have an upper hand on the more aggressive species. Always do your research before adding or buying any fish. Cichlids enjoy alot of Rockwork so that is well encouraged. Most Cichlids you'll see in pet stores are Mbuna which when translated means Rockfish. Try to get a nice stack of rocks with plenty of nooks and crannies for them to call home. Rocks also play a vital part when breaking the line of sight between aggressor and victim. Overstocking is encouraged to spread the aggression, So be sure you have an adequite filtration system.
Substrate options vary slightly but the main idea is to have either a very fine grain gravel or play sand found in playgrounds/childrens sandboxes. When setting up your rocks its best to set them up either directly onto the bottom glass of your aquarium or on egg cartons as cichlids are notorious for digging and could dig under a supporting rock and your formation could come tumbling down onto the fish. I personally used something called Turface Pro-League. Not sure if you can find it in the UK, But here in the states its used as a topsoil for the infield of baseball fields.
Most rocks will do for rockwork, But be sure they aren't going to have an negative effects on your water parameters. Get some white vinegar and put a few drops onto the rocks you plan on using, If they fizz don't use them. "Texas Holey Rock" is a popular rock to use because of its natural holes and cave like sculptures. Sandstone will do but I've heard of it dissolving over a perioud of time (Most say 4-6 years) and some of the dissolved sand getting into the impeller of HOB Filters. I'm going to use Flatrocks for my aquarium, They don't generally look as natural because of the stacking of them, but they do create more places for cichlids to call home. They also create a good place for spawning. When stacking and placing rocks, Be sure you leave adequite swimming room.
Generally aquarist look to fake plants in place of real plants, Most cichlids dig as stated earlier and will dig the roots of natural plants and even tear leafs up. Plants aren't used as much with Cichlids as with other fish. They're quite comfortable with rocks so there's not much need for plants in the aquaria. If planning to spawn them, A ratio of 1M/3F is best.
That covers the basics, Once you post your water parameters and Aquarium size I can further help you on your selections.