Gooden's Nomad Bee (Nomada goodeniana)
It may look like a wasp; indeed I thought it was a wasp but it's a bee, a nomad bee.
The adult nomad bee feeds on nectar and are fond of Dandelions in the spring. The female Nomada goodeniana waits until an Andrena bee female has excavated a nest tunnel and has provisioned a nest cell with pollen for her larva. This will take many pollen gathering trips and the Nomada female will take advantage of the host's absence to visit the cell and lay her own egg in it. Just lie a Cuckoo!
Seems a bit of a lazy approach to life but it obviously works!
Find out more about the nature of Dorset at www.natureofdorset.co.uk