Well, that's the fun of gardening, the best bit I always think! Look at the plant information, check the height and spread details - plant the taller ones at the back, leaving sufficient space between each one to accommodate their spread. Always leave a gap of at least a foot, preferably 18 inches between any fence or wall at the back of the bed and a plant. Red hot pokers are untidy growers, they make a sort of tumbled heap of leaves with flower spikes coming out of the top - they vary in height, so position them with plenty of room between and either behind shorter plants, or in front of taller ones. They also like a bit of sun. A warning with the delphiniums - slugs and snails absolutely love them, especially when they're small, and will relentlessly eat every scrap of foliage till the plant gives up and dies - essential to place slug pellets round them once they're in the ground. As for the dahlias, again, depends on the height and also the colour of the flowers - consider whether you, for instance, like orange right next to red or not. One combination that usually looks awful on the colour front is bright yellow and bright pink - not a great look! Otherwise, most things combine fairly well. You might also want to consider adding some smaller shrubs to the mix - these will give a sort of skeleton framework that doesn't disappear in the winter, even if they're not evergreen. Lastly, remember, if you don't like the way something looks when it grows in the summer, relax - you can always move them around come late September!

Thank you I had a dig about today and dug up lots of bulbs so I think there could have been quite a lot of things in there already. I have moved the old bulbs to another bed so I can organise my new one as you have suggested! Thanks for advice on delphiniums and slugs too- pellets on list!