Delphiniums are really a very easy plant to grow. You do however need to start with plants or seeds that have been raised to grow in your climate.
Most commercially available delphiniums are of the Pacific Giant strain. They were raised in California in the 50s and 60s by a chap called Frank Reinhelt. He did a great job in breeding colour lines which he gave names like ‘Astolat’, a pink and ‘King Arthur’ a purple. The flowers were of excellent form and colour. He did however raise them as an annual. Also, since he stopped breeding in the 60’s the strain has deterioated and has lost the quality of colour and form.
Far better is to source seed from a modern breeder or plants from a reputable supplier such as Blackmore and Langdon.
You can buy high quality seeds from the Delphinium Society
I shall write more in here and include some pictures soon.