Hi Matthew,
My allotment is planned as a potager, so I grow a lot of flowers alongside my veg and fruit and always have a wealth of bees and butterflies. Some ideas:
– for early spring, I let some brassicas (purple sprouting broccoli, kale) go to flower. This provides a source of food for bees early in the season. Limanthes (poached egg plant) is also good for this time of year. For later spring, flowers such as sweet rocket are useful.
– Summer favourites include English marigolds, cornflowers, poppies, sweet peas – all easily grown annuals. Sunflowers are great for bees, butterflies and the seedheads feed birds.
Lavender is a favourite of bees and butterflies – I have planted an edge of my plot with a lavender border and it is always alive with numerous insects when in flower.
Our allotment site “cultivates” a patch of stinging nettles, which becomes a breeding ground for certain types of butterfly. We also have a couple of buddleia bushes which butterflies love.
You can find more ideas by searching the internet for flowers for bees or butterflies. Many seed companies now identify flowers that are particularly attractive to pollinators with a symbol of a bee.
Best wishes